Yes it is possible when driving in DCC mode but with steam engines not possible in CAB mode. To lower the max speed you will have to edit the engine spec. config file not the loco config file. In the engine config file there is a tag for max speed, the figure in the tag is in metres per second. if you lower this figure the loco will be limited to a lower speed but will not take effect unless you remove the existing loco in your session and replace it with the new one. To find the speed you want look here:- http://www.digitaldutch.com/unitconverter/speed.htm.
