Does anyone know of folks who do custom trains? I've got a project in mind.

There are folks who build custom models.  I routinely discourage potential customers - the last custom locomotive I built was for three grand, and it did not have sound, smoke, or Proto-3.  

Here is how to look at it - figure out what you cost your employer per hour, then figure out what percentage of skill it would take, compared to your skill as an employee, to build your model, then estimate how many hours it would take to realize your desire.  Something fairly simple then starts at the $500 mark.

That is why our models are mass produced in China.

The big issue would be fitting the sound boards and even harder, the speaker. Expectations are everything here. Delicacy expected so you can be surprised if it isn't etc etc You could likely slip in a modified conventional MOW drive from ETS or old MTH Volkswagens. Headlights and flashers not too hard. PS3 fitting ??? No clue. The wheelbase on those is adjustable already and extending it didn't look hard to me in the past. Like two 1"x1/2" C channels that could be telescoped on the inside edges for more length . Ive seen and ran 1960s and earlier tin dealers models made in Japan at about 1:24-1:32 with both narrowed axles and with stock wheels simply hovering one with hi-rail pilot/trailer like the inspection station wagon, while a trolly drive or post war inspection car motor did the work well within the frame width. A smaller scale might be better as the width will kñock things down left and right. 1/32-1:43 Id say (or MOW wheels will look HUGE) Of the few I've ran the narrowed ones used the pull-back spring motor frame, bushed, with a worm, or chain drive adapted. Ive always got my eyes peeled for an old yellow Caddy, T-bird and, two ambulances (one had bad teeth?) over scale, but on O .( And there were 2 Lionel inspection cars done as ambulances as well. Maybe detailing an inspection wagon to suit is another option ? On Tandem's site there is a drawing that illustrates closer to the look of the wide "floating" tin dealer model bashes than the actual way a Lionel #68 sits. It looks like a T-bird drawing too But it's also the #68 page so see if you could call that passible fun with some goodies attached...not perfect, but passible. (Id check for a modern one às well, these are likely pricey for a critter) I can only link, no attachment tool today https://www.tandem-associates....e_inspection_car.htm I have bashed some power into small toy VW vans, but for a higher quality runner I like the ETS/MTH mow frame or similar Would you like some drive pictures when the attachment tool works again?

Lionel has made several rail inspection cars and trucks.  I would just get one of those and do a body swap.  Won't have sounds, but at least you have the right look going down the tracks.  There are also the old K-line and now Bachmann autos that work on 3R, E-Z Street.

This is a mighty mite. It pulls two cars or a PW 6wheel crane with some wheel spin; so...needs weight to be more unprototypical diodes are for dimmer headlights at X volts ; I added headlights . Round thing with legs inside is the bridge rectifier for ac to fwd only dc motor, horizontal worm drive. The bigger "hockey puck" is glued weight on the rear frame. The frame is halved and slides, look for the step even with the right side of th  "puck". The frame issue cinched by set screws, as seen below mid puck if you look close. My wires pass though a slide slot... to keep them out of the windshield. With no plugs used, I left a lot of slack wire. The body mount is just two tabs to the bumper area and attaching more angle brackets to the sides wouldnt be hard. So even if you cut a hole iñ the floor and "float" the other wheels, this metal chassis is small highly adaptable with little effort.

Disk pickups; gets better as they wear it seems. Fair on turnouts, a little speed cures all (not light-speed)

This is a couple of bashes of old DVD, CD, phono, toy or tape player parts into cheap pullback toy VW buses This ones a worm drive (cd/dvd tray or linear tone arm motor?)

 The wheels and axles are fast angle, front frames ànd bearing is a split; top half metal, underside plastic. Fast angles dont have the best contact. The axle to frame contact is weak here too. Axle wipers and wraped wire around axles helped the later. Weight helps the wheel to track contact. The rear slots are the same, or some use the original snap-in spring motors gutted frame for bearing and/or motor support. The halved bearings are just horizontal slots. The oblong holes are less than ideal but work with drive belt tension holding the rear from shifting, i.e., no dog tracking unless there is mega-drag on one side.

Flat tread postwar wheels would provide better traction I think but fast angle have splined hubs so only the gear or pulley must be fitted. I add a small flat(s) and a spline mark(s) to the axle and rough the hub i.d. , maybe drill an index notch in gear/pully fàce and JB weld it all, forming a longer support hub tied to the index hole(s) and through, then stake the JBW like a rivet head and cure.

this one was an all plastic frame. The foam tape stops any spinning, locking the motor body on the hole's edges. Finding pulleys smaller than the wheel but bigger than the drives pulley for a better than 1:1 ratio when needed due to motor rpm ranges was not easy on my junk reserve.

the alxe pully just sits below. Clear dental bands drive them. All 4 vans can pull one car minimum , 2 max. and done for the price of old toys some nuts and bolts and zip ties. I expect hundreds of hours from each too... or not... My expectations weren't high, nor cost, nor serious effort for longevity, but they do run pretty smooth... better than 50% of my diesel switchers do

Wow ADRIATIC  that is really cool!!

Here is part of my theory.   

I  do have a model Ecto-1 but its a big sucker.  I'm putting it next to my premiere diesel for comparison.    Its about 14 inches long and a whopping 4.75 inches wide.     The Deisel is about 3 inches longer but of course its only 2.5 inches wide and I'm not sure if the balance could be achieved even if you have the room around the track to do it.

But IF you did I would think you could break down a protosound 3.0 deisel, lose the top of course and rearrange the electronics and gut the bottom of the ecto 1 to make room.  From there you could splice in to the light wire and reprogram the sound for the ghostbuster siren.

 

 

 

 

There is  a 1:24 model mentioned in the OP that might work better.

 

 

 

How about this idea for now.    Mount it on top of the Diesel for this year.  Have someone put in the lights and run them on a battery for now.

The only issue is how to mount it.   I have a 3 o gauge rail and I'm thinking about putting it in the middle but it would have to be high enough to clear the Halloween figurines I've got in the outer rail so if I mounted it and wanted to elevate it a few   inches higher than the top of the diesel does anyone have any ideas?   I'll run it very slow.   Would a form fitting wood piece platform or foam painted black fitting over the loco be an idea?

 

 

Here is what I ran last year.     I could put the pumpkins with the long legs on a flatbed car like the others.

 

Walmart has a 1/43 size ghostbusters ambulance,  about $7.