> I mean, by what measure is rail more expensive?

Your average truck puts out 8 metric tons per axle [1], railroads have to deal with 20-25 tons per axle [2] (and records of 40 tons/axle in Australia [3]). That is an awful lot more force that needs to be distributed into the ground, particularly as trucks usually reach only 80 km/h whereas freight trains run at 120 km/h in Europe [4].

Additionally, road vehicles have way more tolerance for shoddy road conditions - small bumps and potholes won't bother them too much, even at higher speeds, whereas rail vehicles are really, really prone to (deadly) derailments when the track goes bad for whatever reasons.

All of that makes constructing rail way more complex and as a result expensive.

[1] https://en.wikipedia.org/wiki/Road#Maintenance

[2] https://www.forschungsinformationssystem.de/servlet/is/32505...

[3] https://en.wikipedia.org/wiki/Axle_load

[4] https://de.wikipedia.org/wiki/G%C3%BCterzug#Geschwindigkeite...

Do trains really need to be so heavy? I was imaging things the size of cars or smaller on rails, moving around everywhere cars currently move around.

Unfortunately yes. A car or even a truck is standalone and its frame only has to account for the forces of itself moving and crash absorbance, whereas a railway car has to be able to withstand the pull and push forces of dozens of cars - the most basic train has one locomotive at the front and each car must be capable of transferring the entire pull force that the first one behind the locomotive gets, which is for European railways usually anything between 200 to 700 (MTAB IORE) kN.

And the only way to achieve that capability is lots and lots of very thick steel.