Over the last couple of years, I've fitted a number of locos with "stay alive" capacitors, as detailed in the Zimo decoder manuals. At the weekend, I bought a TCS KA2 stay alive unit, and have tried it attached to a Zimo decoder.
Bottom line; it works well, keeping the loco running for 20 seconds or so, covering a yard of track with power disconnected !
The KA2 has two wires, blue (to decoder blue) and black/white striped, which goes to the decoder ground ("mass" on some Zimo diagrams). Compared to the DIY circuit in the Zimo manuals, the KA2 appears to be lacking the inductor (for programming sound and changing firmware) and a discharge resistor. The discharge resistor is probably a good idea, as the KA2 can keep the Zimo decoder memory running for more than five minutes - put a loco back on the track with its memory still alive, and it will run at the last setting it remembers !!
Internally, the KA2 contains a couple of diodes and a charging resistor (to limit in-rush current), and another component (probably inductor, not sure yet), plus six 1F capacitors rated at 2.7v in series. This gives a total of 0.16F rated at 16v, which corresponds to the maximum track voltage which TCS recommend for the KA2. If the shape is wrong, then the capacitors could be removed from the circuit and strung around in a different configuration.
A DIY version would be possible if equivalently compact capacitors could be found from electronics suppliers - I've found US sources of these, but not yet identified an EU/UK source with modest postal charges.
Overall, recommended in any loco which might have pickup problems assuming there is enough space for the stay alive unit. Unfortunately, its usually the smallest locos which have pickup problems, and those often lack any space.