In the mean time, a non-insignificant portion of the population would like a 1-2 liter/100km car so they lower their gas bill.

It's not a "train" since it isn't pulling anything.
It could possibly be used as a railroad speeder by track inspectors and the like if and where the railroads still employe those folks.
But on the other hand, one does have to pull the speeders off the track every so often to allow a train to pass by. That could be a challenge with a sMart car.
if used, it would cost a lot less than what the railroads use now, larger trucks and SUVs.... especially the diesel versions of the sMart car.

Incredibly difficult.
Sure.
Whatever.
Like many other achievements in the production world, the most difficult part is getting the funding, and making an economic case for the "improvement".
Ooops sorry, this came out of the advertising budget, not the engineering budget, no cost analysis needed.

In the country which invented the train they could not design a proper spoked wheel, all what they could do was to put an 80 kg metal slab at the end of an "incredibly difficult" conversion. I'm afraid to check what else this train engineering specialist is doing.

steel wheels and such a short wheelbase? it must have been a very punishing and loud test run.

I think it's rather obvious as to WHY they would 'do it' as can be viewed here - https://youtu.be/mkpCzp0CmjY
Actually, you DO know, this isn't such a far-fetched idea. Imagine commuting to work at a distance whilst sleepy. You drive onto the tracks near home, put the car into 'auto drive' and catch a few Zs while the tracks carry you to your destination. Coming home you could be snockered and still make it home safe - so long as the car knows where to stop.
Kinda puts a whole new slant on the 'shuttle car' doesn't it?

I've seen similarly modified pickup trucks in use on tracks near where I work. The railroad companies apparently have need of autos adapted to run on rails.

Used to drive old chevys on the track all the time. As long as you don't touch the steering wheel all is fine. Just drive away when you come to a road crossing. There have been perhaps thousands of conversions done, first time I have ever seen milled solid steel wheels. Trust Mercedes to complicate such a simple solution.

It is interesting to note that for some time the wheel spacing of many American vehicles was about the same as standard gauge rail. For some that meant letting some air out of the tires would allow travel atop the rails, and of course our railroads use small trucks atop the rails with guide wheels for maintenance. This conversion seems rather pointless, as it has no rubber tires and doesn't allow driving off to complete the last mile. Further, I was expecting they might have tried pulling a wagon or two with it, to demonstrate that it can be done due to rail's low rolling resistance. Maybe MB execs thought it could do that too, having forked over the money for the conversion, but no, the car is way too light, and not recommended for towing even as original.

Lionel had a model of the "train car" in the '50's. Just a regular automobile with lowerable train wheels, front and rear. Drive on or off of track at crossing and raise or lower wheels.