From TTWiki

Limit the speed of trains with freight wagons

2.0.1 alpha 39 or later


Configuration file: wagonspeedlimits on|off|<num>

Command line: -YL


With this feature, most freight wagons will be limited in how fast they are allowed to travel. The lowest wagon speed will limit the speed of the whole train, with some types of freight wagons being slower than others. By default, passengers, mail and valuables have no speed limits.

This feature is really intended to be used with recent train graphics sets, but it will work with older sets or TTD's standard rolling stock as well.

If the switch has a parameter (<num>), then this specifies how much faster empty train wagons are allowed to go. The default for this is 20 km/h.