There is second possibility to get a flickerfree light, which gets along completely without caps. It works only with newer Märklin locos, which bulbs has no contact to the chassis, what is the digital ground.
A decoder has four diodes, which works like a rectifier and convert the digital voltage to DC. The negative proportion will be used for the locomotive lighting. If the brown cable, or the framework of the locomotive are used for the return pipe of the bulb, the light will flicker. Newer decoder like the 60901/60902 has additionally an orange cable, which leads the positive DC voltage. If this cable is the return pipe, the lighting does not flicker.
Therefore you must exchange not all decoders. It is possible to solder such a cable onto the old c90/c80-decoder at the rectifier diodes, but that is not quite simple. Something exercise to solder the small SMD components are needed and additionally you will lose the warranty claim. It it easier to rebuild the positive part of the decoder rectifier externally, as the picture shows. The photo shows the sloppy wired experiment circuit. In the meantime the connecting leads of the diodes were shortened and all cables were laid correctly.
Two diodes 1N4001 are needed. You have to connect the anode of the one diode to the poickup shoe and the other anode to digital ground. The two cathodes are connected together, and this is now the flicker-free back wire. Into reality the current flows exactly differently around, but that is not to disturb us. Main thing it works.