Zimo offer the option of adding "stay alive" capacitors to their chips, which will keep the chip alive when the loco is not connected to the track. This can be a bit of dirt or similar getting in the way. Its a cheaper DIY equivalent to the Lenz "UPS" add-on for Lenz Gold decoders. Lenz use "superCaps" which have massive capacitance and can keep a loco running for several seconds, but superCaps are expensive and need extra circuits to work on 12v.
I have a Simplex in 7mm scale, from an old Impetus kit. Its rigid underframe, 4 wheels, very short wheelbase. As its rigid, there is a fair chance only three wheels are on the track at any moment in time, so its a bit sensitive to tiny bits of dirt.
As there is lots of space inside, I have tried a 4700uF capacitor connected to the Zimo MX63, with the additional diode, resistors and choke recommended by Zimo. This seems to keep the wheels running for about 1/4 of a second before the chip decides there is insufficient power for the motor. The chip stays alive for about 12 seconds before the capacitor has discharged through a resistor. This is a massive improvement in practical running - if a loco has bad pickup for 1/4 second its not going to run at all. It will sail over turnouts where it previously would stutter or stall. Obviously 1/4 second doesn't allow silly stunts, such as running along a workbench, but practical considerations suggest that is an easy way to break a loco !
4700uF is a big lump for anything other than 7mm scale. In smaller stuff, I've tried 100uF and 220uF capacitors and it does help even if its only keeping things alive for tiny amounts of time.