Parts needed:

Passenger coaches (we'll use MTH Overton Coaches)

Scott's Universal LED (the Overtons take 2 per car)
Electrical tape 3/4" x 3/4" x 2 pcs

Tools needed:

Phillips Screwdriver

Soldering Iron

 

Start by removing the 4 screws holding the car's base to its side+top assembly.  These screws are recessed and behind the truck axles.

 

Once the 4 screws are loose, turn the car over on a surface to spill the screws out.

 

With the top off, you'll see the bulb-and-socket assemblies.  Here I'm prying up the whole assembly bulb and all; you can also take out the bulb and then use a screwdriver to pry up the socket.

Here the bulb-and-socket came out ok but one of the wires was left behind: it was a little short and had't actually been soldered into the socket at the factory.  Not to worry: there's a little extra wire under the tan seating deck.  On six cars, I only needed to pull the deck on this one car.

To get the seating deck loose, there's another pair of recessed screws... move the truck axles out of the way as needed... spill the screws out as before.

With the deck up, we can see a little extra wire curving around.  This will come in handy later on.

Push the wires back up through the socket to force out the metal tab and the wire end.  If it's a firm fit, use small pliers to hold the wire closer than my fingertips are holding, to keep the wire from bending sideways as you push up.  If it's really in there tight, use a dental pick or very small screwdriver or whatever tool is within arm's reach to fish out the metal tab.

To de-solder the tab from the wire, place the iron on the tab while gently tugging with the pliers.

Once the solder melts, the wire will pop away.  The wire end retains just enough solder to be pre-tinned.

Slide the socket off the wires once both leads are free.  Save the socket and bulb for just-in-case; you'll have a good use for this stuff in about a million years.

Prepare the LED assembly by loosening the screw clamps.

Push the wire into the screw-clamp while screwing down.  In a classic 3-hands operaton, my pinky and ring fingers hold the white wire while my thumb and middle finger hold the LED assembly.  This allows me to keep pressing the white wire into the clamp assembly while turning the screwdriver with my other hand.

Attach the other wire the same way.  To tighten the clamp screws, move to an easier-to-hold position.

Under the seat deck, bend the wires around as needed to get just the right length on each wire and to position them so the wires are all nice and neat.

The LED assembly sill pretty much slide into place.  Fuss and bother with the positioning as desired.  You can use a drop of hot-glue to keep it in place, but I don't.  If you fuss with it enough, you'll find a couple cleverly tight positions that will hold it in just so.  Or if you fuss with it endlessly, maybe you'll break it.

I've got the thin end (with the LED itself) under-lapping the left edge of the clear-plastic fixture by 0.5mm.  With one hand I'm keeping the middle stabilized; with the other hand I'm squeezing the wide end down into the clear-plastic fixture.  It's a slight press-fit.  Getting it just right is not that hard.

Screw the seat deck in again, then re-adjust the LED's as needed, assuming they got jostled in the process of screwing the deck on.

The cars have some metallic tape to reflect heat and light.  To ensure a loose LED won't short against this patch, put a small square of tape on the roof-spot opposite the 2 clamp-socket solder points.

Here the plastic electrical-tape squares are in place opposite the LEDs' metal posts.

Slide the top back on, taking care that the wires run down their vertical wireways ok.  Put it on the track to test for light, then screw the car back together.

And there you have it!  I don't have any before-and-after pictures highlighting the difference in lighting, so you are pretty much on your own there.  Some say the "bright white" LED's are a bit "blue" but that is because the original incandescents are so yellow.  If you prefer a yellow tint, you can put a small bit of transparent yellow plastic around the bulbs in the cars, after fitting in the clear-plastic fixtures and before putting the top back on.