How can a train unexpectedly be the wrong way round?

This is not really an answer, as David and ptityeti have both covered the reasons why trains may not always have the same “orientation”, and why the announcements may not match reality, but illustrates a few more related situations (but is too long for a comment).

• Eurostar trains are (or at least were, not sure about the new e320 train) fully symmetric (they are actually composed of two identical half-sets), with “Premier” coaches in the middle and “Standard” coaches at the ends. So even if they get spun around at any point (AFAIK, the only reason for them doing so would be to have a train do an out-of-service Brussels/Paris trip, or a more exotic Marne La Vallée/Paris Nord), they can just switch the numbering of the coaches (which is of course electronic), and voilà! it’s back to normal. The #1 coach is always at the London end.

  Strangely enough, when picking a seat on the Eurostar website, they will tell you that for some seats they can’t guarantee which direction they will be facing. Not sure if that was a very, very early anticipation of the e320s being introduced, as the composition of the e300s is really very well determined. Maybe the refurbished (ahem) trains have a different layout?

• in France, TGV trainsets usually have 3 coaches of 1st class, the bar, and 5 coaches of 2nd class (though the exact composition varies a bit, and of course TGV Atlantique trainsets have a total of 10 coaches). In many cases, it would make sense to always have the 1st class coaches at the Paris end (as they are all terminal stations, while most -but nor all- other stations are through stations), but on a given line, you can see both orientations, even for a service at the same time on different days. And with coach-to-platform position indicators on the platforms, you would expect that the train actually matches what is advertised prior to arrival (and it usually is), but I’ve seen instances of reversed indications which lead to the obvious mad dash (especially as TGVs have compulsory seat reservation).

• some trains, notably the TGV Réseau trainsets have double-numbering of the seats. When you book, you can choose whether you’ll be facing the direction of travel or not, and the numbering inside each coach is designed so it can be reversed to match (all seats have two numbers, and only one set is illuminated). That means they reverse assignments within a coach, but the order of the coaches may be different (which is different from what happens on Eurostar trains).

• interestingly enough, other recent trainsets, such as Duplex trains, have electronic seat number displays, so you’d think they could reverse seat number assignments, but they don’t provide the “facing direction of travel” option.

Another common reason why trains don’t have the expected composition is a last minute change due to some malfunction of the scheduled train. In such a situation the priority becomes often to have a working train – any train – rather than making sure that the position of the coaches is as expected.

I’ve even had a case where reservation was mandatory, but the scheduled train was out of order. They replaced with another train, but some people had reservations for seat numbers that didn’t exist on the replacement train.

Also the electronic displays on the European continent are not always correct. I’ve recently had a train in France where the numbering of the coaches the inverse of what was shown on the electronic (so easy to correct, I’d think) signs. It also resulted in people running from one end to the other in both directions. I’ve never known what was the reason in that case.

A train can end up the wrong way around by going round a triangle (essentially, doing a three-point turn). This can happen in a variety of ways.

1. Some routes call for a train to stop at a station to one side of the main route, which requires a reversal to continue the journey. For example, the station might be a little way to the east of a north-south line. A northbound train will take a right turn to get to the station, then reverse and turn right again to get back onto the main line: the effect is essentially a three-point turn. If not all trains stop at that station, some will be reversed and some not. Even if all the trains call there, trains that have been running on some other route for half a day and then move onto the route via with the reversal might have been turned around a different number of times to a train that’s been running via the reversal all day.

2. The same can happen with the maintenance depot.

3. Occasionally, trains get diverted via unusual routes which may require reversals.

4. Trains from A to B and trains from C to B might both pass through station D. Because of reversals or other reasons, the trains from A might be consistently oriented in the opposite direction to the trains from C. Perhaps the announcer just made a mistake.

As to how it can come as a surprise, I guess they just don’t pay much attention to the actual orientation of the trains. On some lines, almost all the trains are oriented the same way. For example, the London to Edinburgh line has no opportunities for turning trains so they’re essentially all facing the same direction. If one did get turned around for some reason, the automated announcements would probably just plough on regardless because it’s such a rare situation.

In general, there are usually signs saying where each coach of a long distance train will be if the trains arrive in a consistent orientation.