Topt speed in DCC mode is defined in the motor section of the enginespec. Here is a snippet from SD40-2 enginespec,<kuid:-1:42004221>:
motor
{
resistance 1.7
adhesion 2.3
maxvoltage 600
maxspeed 35
brakeratio 55000
The maxspeed value is in meters/sec. 35 m/s is 78 miles per hour.
Maxspeed in cab mode is determined by the throttle-power section for diesels or the steam section for a steam loco. Some info on this is in the wiki at
http://online.ts2009.com/mediaWiki/index.php5/Steam_container, however, I think it is currently down with a problem.
Bob