[SRman]

OP has already specified it as being Bachmann. If it is an earlier issue it has a switch underneath that alters the headlight configuration from daytime to nighttime but if it is a more recent issue, the switch switches the tail lights on or off but the headlights remain permanently in the daytime configuration. In other words that switch will not affect the behaviour of the decoder at all.

As with the other posters, I'll await further info on what decoder you have installed before suggesting anything else.