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Is it reasonable to re-state the objective as to avoid rear-end collisions?  If so, some kind of block control can stop/slow a trolley that's gaining on its predecessor.  In fact it would be interesting to work this into some kind of trolley station-stop configuration if you have stations along your line.

Adding speed control electronics to a trolley is in the realm of possibility but not an easy project.  And even if you could achieve, say, 1% absolute accuracy that still means after some running time there will be a rear-end event.

I suppose a separate approach is to forego absolute speed accuracy and insert components in the electronics to adjust motor voltage for relative accuracy - that is, so that they run at about the same speed.  Again, after some time there will be a rear-end event.  And with grime build-up in the gearbox you'll be spending more time in maintenance than revenue operation!  In my opinion that is!

Assuming the trolleys are identical except for the paint scheme, off the top of my head I'd say a combination of:

1) mis-match in the motor.  These are typically low-cost DC can motors where the torque vs. speed curve varies 10% or more just from variations in assembly, magnet strength, etc.

2) mechanical mis-alignment or imperfections in gearbox/transmission.  This can include variations in wheel slippage; wheel slip occurs more often than you'd think.  Traction tires can help but introduce another source of variation.  As an aside, this is the folly in adding speed-control electronics which regulates motor speed rather than actual track speed.

3) grime build-up in the gearbox/transmission.

@shorling posted:

My EZ Street taxis ran at different speeds.  I adjusted their speed by placing diodes in series with their DC motor.

I added blocks to prevent collisions.  I also the slowed the speed in the blocks by adding diodes in series with the block power.

Thanks for chiming in.  My original suggestion was some kind of block control (e.g., station stops) to prevent rear-end events.  But you'd still have speedy trolleys zooming to the next station and then stopping...the old "hurry up and wait" effect.

So, as Steve suggests, you can additionally modify the trolleys themselves with inexpensive components (10-cent diodes) so they run at relatively similar speeds for a given track voltage.  I'd think this would be more appealing.  If not wanting to mess with tiny electronic components, diodes, soldering, etc.. I wonder if adding some lead fishing weights to the chassis of the speed demons could slow them down?

In my case, I had 4 taxis.  Three ran at the same speed, the forth was Mr. Zippy.  I tried weights but it didn't work on the light weight EZ Street taxi.  I concluded it was due to motor variation as Stan stated in item (1) above.  I' guessing Mr. Zippy was from a different lot of motors.   I bought Mr. Zippy as a separate purchase from a different retailor.  The other 3 taxis I purchased on single order and most likely came from the same manufactures shipping carton.

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