This patch allows you to increase or decrease the maximum amount of trains per player in the game at any one time. This setting can be found under the "max_trains" setting in opentdd.cfg, with the default being 500. It can be changed in increments of 100, from 0 up to 5,000 (which is more than enough for anyone) If you change this setting, say, to 100, when you already have 101 trains in-game, then the game will give you a 'too many vehicles in game' error.

Click on the value to enter a particular number

Max Train option in Patch Settings: Vehicles