I took a look at this because I have an MTH observation car of roughly the same generation as yours and I have added LEDs to Lionel observation cars to better illuminate the markers and drumheads.
I can't help on the MTH part # as I can't trace one on their (inadequate) online replacement parts list. However the MTH observation car I have looked at does have brighter marker lights than your photo indicates is the case with yours. The ones shown in your photo appear to be incandescent not LED and as far as I can see the same applies to mine, which also has other incandescent lighting (the overhead compartment lights) in it.
I can't work out what might be causing yours to be dimmer than you'd expect. I will say that in mine the marker light bulbs are right up against the back of the marker light bezels. Lionel made a lot of observation cars where there was no separate light source for the markers and they relied on the interior lights for such illumination as the markers had. It is relatively easy to provide LED lighting for these provided you have a power module that will take AC track power and convert it to DC output. I swear by GRJ's passenger car lighting kits, which I have adapted for this particular purpose.
