The steam locomotives 044 (37880) and 043 MHI (37882) have the socket for the rear light mounted onto the 6090 decoder. This is the reason, why it is more difficult to make the lights flicker free.
On the backside of the decoder there is a large ground contact strip / track (right on the photo). It is connected with one of the socket pins (green arrow). This contact strip / track must be cut (blue arrow).
On the left side of the decoder there are two solder pads. One is ground (brown arrow) and the other for is power/data (red arrow).Two diodes 1N4148 must be soldered on there with the anodes. The cathodes have to connect together and this is now the flicker free wire (orange arrow). A wire connects the diodes and the now ground free pin of the bulb (green arrow). The brown wire of the front light must be removed from the motor shield and also connected with the diodes (orange arrow).