I have some passenger coaches and freight cars from my long-ago analog layout which have internal/external lights powered from a pickup slider under the car. They don't light when running on my new digital layout, except oddly enough sometimes if they are derailed.
Is there any simple way that the lights can be made to work on a digital setup, or do I have to remove them and replace with light sets designed for digital use?

If using a digital controller like the older 6021, there is a risk that the constant illumination will cause the lights to produce more heat than they do in analog operation where the light intensity depends on the transformer setting. This can cause the plastic light channels to melt under certain circumstances. You can reduce this risk by replacing the bulbs with ones that have a higher voltage tolerance.

I have not had much experience with using older lighting with the new Mobile Stations or Central Stations. perhaps Tom (HO) or one of the others with more experience with that system can provide a better response.

In theory, the bulbs should continue to operate normally. The intensity of the bulbs would depend on the tolerance of the bulb. I would inspect all of the connections to check for signs of heat, as resistance will be manifested in terms of heat, which can melt plastic. Make sure that the wires and sockets are properly connected. Those little fold over tabs on some lighting kits that hold the wires to the terminals are notorious for weak connections, as they often bend or break if reused/modified.

Regards

Mike C

Thank you all for confirming that it should work. I have tinkered some more and can see now that is almost certainly loose wires or dirty connections somewhere. No wonder, they have been sitting in boxes in the cellar for more than two decades.