Turns the electricity generator of a steam loco on or off.
For the command to work the loco script must have a message handler responding to Major Msg 'service_loco', Minor Msgs 'generator_on' and 'generator_off'.
Two loco assets on the DLS that have scripts equipped with the required handlers are:
<kuid2:44090:1431:11> NSWGR C35 class loco
<kuid2:368725:15035:9> NSWGR D53 standard goods engine, weathered