Answering the specific question:

would it be physically possible to drive from [Germany to Thailand]

What I mean is that: Are there any roads connecting mainland Europe … with Thailand

The simple answer is "yes", there are two ways to go:

1. The normal simple route (blue) is you just drive through Russia then China. (The "high road," if you will.) It’s that easy. Hop in your car and turn on the GPS.

2. Alternately you can take the "low road" shown in red. There are any number of variations and you pass through 10 or so countries.

Driving conditions

You pass over no water. It’s generally flat.

On the normal (blue) road all roads are sealed and straightforward.

Could it be your question is about The Himalayas?

The question "is Berlin connected to Bangkok by land" is trivially answered by glancing at a globe.

For young people reading, I highly recommend opening "google maps" and if you zoom way out you can get a good understanding of this planet.

However, notice the area marked in pink. If you go there you will find a really, I mean really, big mountainous region of this planet. It’s basically not possible to drive through it.

It could be that the sense of your question is basically "can I drive through the Himalayas?". Answer, no, you just go "above" them, ie drive through Russia and China.

Addendum – paperwork

Current political conditions (2023)

Blue route:

As of writing, Russia (the large country at the upper part of this planet which defeated Nazi Germany 80 years ago) is having a war that is apparently disapproved of by some/most political figures in the USA.

Every single web reference I can find states that it’s still perfectly possible to travel to Russia (random example)

OP is not a USA citizen so the issue seems irrelevant.

(If you’re a USA citizen and reading this, it seems the USA government has put up many pages on the web stating that they "recommend" USA citizens don’t travel to Russia currently. However, every serious reference I can find states that travel to Russia (for US citizens) is perfectly possible and normal (random example).

Red route:

You would have to travel through the regions of "Chaos-stan" as it’s sometimes called.

This is a full-on chaotic war zone with irregular and ad-hoc social and political structures.

China paperwork addendum:

It is notoriously a HUGE paperwork fuss to drive in China and/or take your own car in to China.

It’s perfectly possible, and people do it every day, it’s just a huge, huge, fuss.

Road conditions

Obviously, China has far and away the best roads on the planet.

They are so far ahead, and getting further ahead every month, there isn’t even a second place.

(Much as Germany had the best roads on the planet in, say, the 1950s [they are now all falling apart] and the US had OK roads in say the 1970s [they have all fallen apart].)

(Also, of course, obviously, China is far and away the world’s biggest car maker and, if I’m not mistaken, the biggest purchasor of cars. It is a completely car-obsessed country, it’s the #1 "car nation".)

There are any number of major TV documentaries about the wonders of driving in China, including say

https://www.youtube.com/watch?v=4-XDxCb92X4

(I encourage you to watch this if you’re unfamiliar with the roads there.)

(Note that that documentary is very out of date, roads in China are now another huge leap ahead.)

Actually, in the same show they take a number of foreign cars in to China (as well as driving local cars) so you can also see that process.

Summary

1. The question at hand is about the physical connection. Since, obviously, Germany/Thailand are on the same landmass for the last few million years, answer is "yes".

2. Hence, I believe the actual sense of the OPs question was perhaps about "The Himalayas". You can not drive through them. You go either above or below them.

Geopolitics issues: the "red" route goes through the planet’s current "region of unrest", in short "an anarchic war zone".

addendum sealed roads. It’s commonplace to drive across Russia. I understood that it’s all sealed, but it’s unclear, some of the road may be unsealed. IDK. You can google any number of articles "drive across Russia" [example][4]

Yes, in theory it’s possible and it has been done. There was a famous bus ride that departed from Salo, Finland for Bangkok, Thailand back in 2018. The bus made it to Bangkok and it was possible to buy tickets to go on this trip. Obviously things have changed since then, since the routing required driving through Russia, which would be next to impossible these days. The blog can be found here.

Answering the actual question asked:

Are there any roads connecting mainland Europe, like Germany, with
mainland Asia Thailand?

Yes, there are.

Google Maps will refuse to calculate a route from Germany to Thailand, but you can get it to give you a route from, say, Frankfurt to Teheran as well as one from Jalandhar (India, just after the border to Pakistan) to wherever in Thailand you want. The stated driving times for these are around 50 and 80 hours, respectively. If you drive 8-10 hours a day, that’s just over 2 weeks.

The part between Teheran and Jalandhar does have major roads, at least on the map. Also, none of the Google Maps routes indicate that you need to take a ferry somewhere. So yes, there are roads and you can, theoretically, drive from Germany to Thailand without leaving your car (well, except for pumping gas and visiting the toilet).

You CAN get a route from Openstreetmap:

https://www.openstreetmap.org/directions?engine=fossgis_osrm_car&route=50.111%2C8.682%3B12.937%2C100.886#map=4/33.91/54.84

Which shows up as 12323 km and 145.5 hours. It’s pretty similar to what I wrote above, bypassing Teheran on the highway and going through Jalandhar.

The northern route (calculate in 2 parts, Openstreetmap doesn’t do intermediate stops) could be Frankfurt-Omsk-Pattaya. That route bypasses Ukraine, where, to put it mildly, some of the mapped roads may not exist anymore.

Whether or not any of that is a good ideas has already been answered extensively in other answers.

It is actually possible. Definitely not easy but feasible. A friend of mine did this a little over 3 years ago, driving from Paris to Kuala Lumpur, passing through Russia, Mongolia and China. It took and her family about a year to explore the way and she said it took almost that long to plan it! The trip is documented online – in French – on her blog here. This is a rare adventure and they were even met with a TV crew part of the way from Russia to China which can be seen here.

The plan relies on traversing a significant portion of Russia to avoid some of the dangerous countries south of it. The path can be shown on the blog and there are several pages that explain the planning process. The greatest difficulty is timing since most visas have a validity period and a stay limit. Given that these visas also have to be stamped onto passports, it was sometimes necessary to stop near a border, do a visa application locally and then move through the border. China was particularly complicated given that they had constraints of having to go through an agency and they were met by someone in China that escorted them while driving through until Malaysia, crossing Thailand on the way there.

There are roads the entire way and although varying in quality, they managed to cross using an RV. A regular car should have no trouble following the same path. In fact, the RV complicated matters because some visas ask for stay location and hotel information.

Note that for the return, they did not want to retrace their steps and so opted to come back via the South. As mentioned already this is rather difficult and risk proposition, so the opted to ferry the RV from Malasia to Dubai and returned driving to Paris from there (after another short ferry to Iran).

The route through China might be difficult for cars in terms of getting permits but you can definitely consider motorbikes or bicycles.

India to Europe has been done very recently too, like these guys going from Bangalore, India to London, UK through China and Russia

In order to get from Europe to Thailand by car, you would have to pass through at least one of these three countries: Afghanistan, Iran, or China.

• Afghanistan: extremely dangerous and unstable. Out of question.
• Iran: unstable, unpredictable, and under international sanctions. Additionally, a journey through Iran would take you into countries such as Pakistan and later Myanmar (provided you manage to cross all the borders), that are also unstable and dangerous.
• China: severely restricts entry of foreign-registered motor vehicles, and also doesn’t accept foreign driving licenses (https://en.wikipedia.org/wiki/International_Driving_Permit). It’s possible to obtain a provisional Chinese driving permit, but that can’t be requested at a road border checkpoint, it must be requested in advance in China (e.g. at a major airport). Getting an entry permit for your car requires special arrangements, takes a lot of time and energy, and those who obtain it must be accompanied by a local Chinese guide at all times until they leave the country. Additionally, China still (as of Feb 2023) doesn’t issue tourist visas.

To sum up, driving a car from Europe to Thailand is next to impossible. It would be a remarkable feat requiring an extremely capable and resourceful traveler. That’s why most casual adventurers from Europe would end their road trips in Vladivostok, Russia.

Alternately, I’ve read about people who travelled from Europe to Southeast Asia via China by bicycle. Bicycles need little to no paperwork and cyclists need no driving licenses. This is not for everyone, but it’s doable. Cycling can of course be freely combined with train travel.

Borders and safety are going to be a big problem.

I have done a "tour" that went from Katmandu, Nepal, through New Delhi, India (where we joined it) to Tehran, Iran–it continued on to London, England but that’s where our plans went a different way.

Interesting in 1975. Crazy unsafe (Pakistan), probably impossible (Afghanistan) and certainly impossible (Iran–we have US passports) now. The Afghanistan portion could be routed around, but Pakistan and Iran remain big problems. This was a bus, if it could get through any ordinary passenger vehicle should likewise be able to.

There are roads and famous journeys using them so it’s possible in that sense (there is nothing like the Darién gap on the Eurasian landmass). Customs (for the car) and visas will not be trivial to sort out but I have heard about people managing that, even as recently as the late 2010s (either using public transportation or with a motorbike). The main problem will be finding a route that is open and safe, depending on your citizenship.

The classic route from Europe to India goes over Iran and Afghanistan. It was popular from the 1950s to the 1970s but became very dangerous after the start of a long series of wars in Afghanistan. Iran was better but it is currently under international sanctions, experiencing a lot of turmoil and generally more dangerous that it has ever been in my lifetime. Afghanistan has always been challenging and has probably gotten even more dangerous lately. Even people who relish the lack of infrastructure and the instability ought to be concerned with the high risk of kidnapping for ransom. Pakistan is open (actually have a friend who went there for a short visit a couple of months ago) but the border areas are considered more dangerous so overland travel does not seem realistic.

For South-East Asia, you could consider going over Russia, Mongolia, and China but not with your own vehicle (see Johnnyjanko’s answer about that). I think all these countries are technically open for visitors. Relation between European countries and Russia are tense but it is not necessarily in the “do not travel” category just yet. One question is how to get to Russia from Germany. The border with Ukraine is obviously closed, not sure it’s possible to reach Belarus from Poland. Baltic countries have been talking about banning visitors from Russia but maybe you could still cross the border in the other direction or go all the way to Finland?

Further East, I believe China has officially reopened its borders to visitors after a long freeze in relation to the Covid pandemic but I am not sure there are many visas or visitors just yet. Some regions also require additional paperwork making overland travel more difficult. Myanmar is completely off-limits so going overland from India is impossible and Laos is the only options from China (or Vietnam but it does not share a border with Thailand). There is also one border crossing between China and Laos that may be passable.

Compared to all these countries, Thailand is safe, open, and easy to travel to. I know several people who visited recently. They all came by air but overland travel has historically been possible and I don’t see any sign that its borders are closed.