From a forumthread:

The basic train with maximum speed 80 km/h takes exactly ONE MONTH to travel few over 80 squares of the map. Such a train takes two years to travel whole 2kx2k map from one corner to other (not diagonally) and 4 years to travel there and back rendering it useless for such long trips. As you can see, one square of the map is considered to be 1 km, so the speed of the train tells almost precisely, how many squares the train can go in one month at full speed.

Keeping this in mind and using basic math you can count how long the train can go per year to stay profitable. For our basic 80km/h train this means going some 1000 squares/kms (12*80=960) per year max. If the train does not need to wait long for getting loaded, which should definitely be the case on mainlines and long distances, then such a train can go in distance of roughly 450kms and back in one year, which is his effective maximum.

For very huge maps this means, that trains of speed around 80 km/h can go at most 1/4 of map to be effective. Trains of speed 160 km/h can go half map to maintain efficiency. Trains going over 200 are suitable for 2/3 to 3/4 of the map size. And only trains with speed of 320 and more can go whole map there and back in one year and thus remain efficient.

Read more here in this forumthread.