A while ago I built a cheap but effective (ish) PWM controller using a motor control PCB from ebay, a DPDT switch, a project box and the sockets needed to accept a 12v DC input and a Kato track power supply socket output. I say ish as some motors won't actually stop with the knob turned right down!   Since then I've acquired another PWM module for less than a couple of pounds on ebay and read up on 18650 batteries. Which led to this test rig:   The green power bank and USB lead will be replaced when the next part of the puzzle arrives. I've ordered an empty power bank casing which is designed to hold and charge up to five 18650 cells in parallel, by only fitting three or maybe four I should free enough space to fit this extra circuit, which I'll patch onto the USB output pins of the power bank circuit board. So far it draws one or two milliamps when powering a Bandai DD51 hauling four coaches, so even if I can only fit three batteries that should give an impressive runtime. It'll also double as a power bank, just one with an extra switch, knob and socket at the opposite end to the USB connections.   As for the train refusing to stop, this one has a positive "off" setting with a click! The train starts moving at about halfway around the dial so the click has no impact on fine control.  
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