Ed Mines wrote:

The bottom valves on steam era tank cars are way too small to allow
the car to be emptied in a timely manner.

Where do you get this idea? It was done very widely and all the
time. And photos I've seen of top unloading pipes are no larger than
the outlet pipes at the bottom.

I have a feeling they were difficult to open too; otherwise thieves,
vandals, hobos etc. would be opening them.

Nope, the accessible part is just a pipe cap on the outlet
pipe, but that's not enough to unload. You also have to open the
valve, which is INSIDE the tank and operated from the dome. See any
steam-era Cyc or a book like Ed Kaminski's _AC&F Tank Cars_ for
drawings.

I think tank cars used to transport ashphalt, roofing tar etc. were
the tank car equivalent of hide loading box cars.

Not at all. Tank cars could be and were cleaned for new
cargoes. Of course it cost time and money, but it was certainly
possible. The major exceptions were cars with particular linings, such
as rubber linings for acid, which you would not want to use for
anything which could interact with the lining.

Tony Thompson Editor, Signature Press, Berkeley, CA
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