How in our modern world are people painting passenger cars, locomotives and other large objects... In modern times we are unlikely send them out into the world to a well equipped shop... Air quality regulations are increasingly limiting paint options. Are there any vendors offering on site painting?

Still brush painting by hand or roller. Would like to spray paint, but the people with the experience and equipment fail to materialise.

The spray paint that's worth the effort is the "dangerous" two-part epoxy/catalyst stuff for which the air quality laws were written.

If you're spray-painting cheap latex, you might as well get the rollers out for all the good it'll do.

I have witnessed disused warehouses with enclosed rail sidings get set up for "under the radar" painting of rail equipment with "quality" paint, and I am aware of one railroad loco shop in Illinois where several locos and passenger cars were repainted "on the sly" not long before the entire facility was demolished and leveled completely. (I'm not sure that was exactly the plan, but that's what happened. Supposedly at least one loco and car were damaged by parts of the roof falling down!)

There are some mobile railcar and locomotive painters as well as industrial painters. Search online and they’ll pop up. Cost will depend on what type of coatings you want. There are low VOC options that can be applied with HVLP equipment to keep the overspray minimized. We’re currently in the process of having two cars, a caboose and flatcar, blasted and painted outdoors by a local mobile outfit. I’ll let you know how it turns out if you’re curious.

Industrial painting contractors are pretty much universally available, but you might not like the cost or the result. remember, these guys make a living putting highly durable coatings on tank farms, etc. where esthetics take a back seat.

Generally, I don't think the method of application matters that much relative to quality of surface prep and assuring you use compatible high quality products. I have had good luck with PPG water based industrial enamels brush and roller applied where environmental considerations and overspray control were critical.

I directed the painting of a former Philadelphia and Western transit car. It was a brush and roller job, using quality equipment and tractor enamel. Worked out just fine and still looks good 20 years later.

Nothing wrong with a skillfully applied brush job. You'd be surprised at the number of companies that brush painted their cars back in the day... Such as the North Shore Line. We were debating this years ago, and someone familiar with the North Shore walked over to one of the cars that hadn't been repainted since the line quit and was well oxidized, lightly burnished the oxidation on a portion of the side, and the brush marks became faintly visible.

I haven't painted anything at the museum in years, but based on my experiences with maintenance painting around the house, The VOC regulations have ruined the paints available to the public at large. It just won't level, and everything comes out looking like it was painted with a rake. Yet there are still paints available in other markets, such as marine paints, that work well. We've discussed Awl Grip marine epoxy coatings here before, and as I recall they are available in a form suitable for brush application.

Perhaps there are coatings products made for tank and facilities maintenance applications that would also be adaptable.

When Old Pueblo Trolley repainted a streetcar, we would use professional grade automotive paint and spray guns. When we repaint a bus, we do the same but use a paint booth at a local body shop that paints RVs. We have the paint made to our specifications to match the colors originally used by the transit companies. Painting is done by our volunteers using our own equipment.

Small parts are repainted in our shop, since we have a proper paint and drying booth.

Within the last couple of decades I saw an article in the British railfan magazine The Railway Magazine that showed and talked about mainline shop painting -- it was being done by brush. And the reason was to avoid the problems and requirements of spray painting.

Somewhere up north, there is a streamlined NP round tail ob that a painter from an auto show just painted recently.
He was on Iron Resurrection for several years and put out the most beautiful auto paint jobs ever. His nickname was Shorty, or Shortdog and I've seen a couple of photos of portions of the car and it is amazing.
I haven't seen any photos of the finished car yet, last one was with the windows masked and lettering done. I'd like to see what the finished product was.