But going with the grain would suggest biomethane for heat and renewable electric power for traction.

True, and renewables are the aim, but I'm interested in the transition, and making best use of the carbon we have.

Now, I'm not an engineer, but I am interested in the "well to wheel" calculation, where we see gas being burnt (with no CHP) and then distributed to run ultra heavy trams around, which need expensive and unsightly cabling and (eg the horrors in Edinburgh at the moment), massively expensive and time consuming work to implement. I have no idea how that stacks up, particularly in climates where ther's no use for the heat.

I must say I'm attracted by the idea of a generation of ULR powered by (say) CNG to begin with, and biomethane to follow. They can run on both conventional tracks and tram tracks as well, so it's not a case of either/or.

It just seems - to a layman - a pretty painless way to roll out a simple but effective transitional solution.

For third world countries, particularly energy wasters like Iran and  Nigeria, it's literally a no-brainer, I would have thought?

And it's not particularly high tech, either, so developing countries could probably build them, with a bit of licensed design IP and quality control.

"The future is already here -- it's just not very evenly distributed" William Gibson