Since my tutorial on Coach lighting, it's been pointed out that sometimes the real problem is actually getting into coaches in the first place! Point taken, so I'll go through the coaches that I have on my layout (this is 00 gauge, so unfortunately I can't offer any help to other gauges folks may be running, but they may be similar). One of the likeliest first culprits will be the early Triang/Hornby coaches, and one of the easiest too as they're probably the best part of 50 or so years old and represent early production line techniques long before the advent of clip together parts. Upending the coach, you'll see a couple of brass screw heads central in the underside, remove these, then looking at the "corridor connector" at each end, near the top lip, you'll see a small slot with a tab protruding from it. Gently press in each tab in turn, and the roof pops off, then the sides fall away to reveal the basic chassis. After removing the seating/partitioning assembly, under this on the chassis is a fair sized rectangle with a small metal weight. I find that most rolling stock of any description is way too light, so I normally add a suitably sized piece of lead roof flashing, removing the moulded bosses from the roof fixing screws as I tend to glue on the roof after modification, as (assuming you make a decent job of the soldered joints on the lighting setup) you won't need to remove the roof again. Since you are adding lighting, it follows that you will notice the interior of the coach much more, so it makes sense to paint or similar the seating/partitions etc and add passengers. I then glue the sides back into position as this adds strength to the whole assembly, before doing the lighting setup (see my How-to on this) and finally gluing on the roof after of course testing the lighting operation. I use exclusively 'Kadee' couplings on my layout, so as part of my modifications, I fit these, which can be easy if the coaches have NEM pockets (well done, Bachmann), but you may need to experiment with older coaches which don't have NEM pockets.