Limiting speed of trolley line

To a point, but it would get hot.  If trolley derailed & munchkins left power on, the resistor could get dangerously hot.   A solution might be an HO DC powerpack, possibly using external diodes to cut the voltage down

I'd consider the diode pairs in series with the transformer output to limit the speed.

Another innovative way is to simply put a removable stop on the throttle lever, we did that for a display for kids to run.

One step-up from the bump and go trolley,  There are control systems designed for trolleys that are not bump and go. 

Dallee systems.    I had installed one of these systems, Fort Pitt Highrailer's town module. 

Slideshow

As others suggest, consider the diode method instead of a resistor/rheostat.   The diode method keeps the voltage relatively constant.  The rheostat will starve the handcar of voltage when turning direction because the motor is "stalled" and that's when it demands the greatest current.  Thus, you will have to set the running speed higher which sort of defeats the purpose!

BTW, RJR brings up an excellent point about a derailment where the wheels short the rails.  With a resistor/rheostat, the full track voltage is now across the resistor/rheostat and the train transformer (e.g., your PW ZW) may have enough power output (over 100 WATTS! ) to create a room space-heater without tripping its breaker/fuse/whatever.   That's going to be one HOT resistor/rheostat.  OTOH, because of the way diodes "work" if the full track voltage is applied across the diode/bridge dropping mechanism in a derailment short, in most situations it will trip the breaker/fuse/whatever.  No guarantees but, again, given a choice I'd go with the diode method.

In any event, you can always add a so-called resettable thermal fuse (50 cents or so).  I'd recommend this with either the resistor/rheostat method or the diode/bridge method if you have a "powerful" (like a ZW) train transformer operating a relatively low-power load like a trolley.  

I use a https://www.tandem-associates....ns_s_transformer.htm  to power an MTH bump-and-go at the A&M museum.   I installed a small self-adhesive clip to prevent the munchkins from powering it up too far;  the clip sticks to the face and keeps the knob from turning beyond a certain point...   

Mitch