stan2004 posted:If you can attach a conventional transformer to the layout, run the offending steamers in conventional with speed control TURNED OFF (whistle-bell-bell followed by the 2 toot acknowledgement). This applies track voltage "directly" to the motor without speed control. If the engine still slows down then the engine is receiving a reduced voltage or possibly intermittent contact which effectively averages to a reduced voltage. I suspect you were measuring track voltage on the rail? What would be useful to know is the voltage the engine gets through the roller/wheel connection; this could be a hassle so the above experiment is a proxy.
Stan - thanks for this! I'll try to do this later or tomorrow and see how it goes.
Peter