From TTDPatch manual:
Now, my question:Another small benefit of additional engines is that they increase the maximum speed slightly, because there is some excess power available to accelerate the train to a higher speed. The first additional engine adds 20%, the next one 10%, and a third extra engines add another 5%. After that adding more engines will not increase the speed any more. The amount of this increase (in percent) is an option to the switch, the default is 35% in total. If you want the game to be more realistic, you should probably disable the speed increase using multihead 0. Otherwise, the parameter can be anything between 0 and 100, in which case the additional engines add 4/7, 2/7 and 1/7 of that amount, respectively.
I found in Bananas two files:
Should I load both to get Vactrain working?