score:7
According to Wikipedia, this is either Nice-Moscow line (3315 km), closely followed by Paris-Moscow (3217 km).
I would have thought that the Venice Simplon Express on the route Prague-London or the Paris-Istanbul route would be a good candidate too, but I can't really find the distances. A google maps route plan using a car suggest that both are shorter than the above, but obviously this will depend on the exact route taken.