"ed_mines" asked:

Weren't most tank cars used to transport petroleum and petroleum
based products?

In the steam era, yes. The overwhelming tonnage shipped in tankcars was
petroleum products. After WW2, pipelines quickly killed that business in
many areas and there was a lull in tankcar construction and development
until 1959, when the domeless non-pressure (ICC-111, later DOT-111) was
introduced. Even then the thrust of new designs was around fuels, namely
LPG. After the introduction of the stub-sill tank designs in the early
'60s, then the plastics revolution, and finally deregulation in 1980,
tankcars were built in large numbers to carry all sorts of stuff that
either didn't exist in the steam era or wasn't economical to ship in liquid
form.

There were many non-petroleum products that were shipped in tankcars in the
steam era, but the total tonnages were relatively small. For instance, if
I remember the 1950 numbers correctly, there were less than 5,000 carloads
of corn syrup shipped, or about 14 a day. Today there are bottling plants
that use more than that a day.

If I were to guess, I'd say the next biggest commodity group after
petroleum in the steam era was acids and bases, especially sulphuric acid,
which was/is used in a lot of industrial processes.

Weren't the tanks used to transport nonpetroleum based products
dedicated to just one product or a few related products?

A little kerosene in gasoline might be tolerated but any petroleum
product in corn syrup would ruin it for food products.

Generally yes, although the car owners probably hated that fact of life.
But contamination wasn't the real problem in most cases, it was car design.
Dump sulphuric acid in a general service ICC-103 and you'll have a big
puddle of acid on the ground. A car designed to haul sulphuric acid needs
a special lining (in effect they're a rubber bottle supported by a steel
shell), so you don't want to dump something into the tank that might damage
the rubber lining. On so on.

It seems to me cleaning the inside of an tank car would be a major
undertaking.

Depends on the stuff it carried and the general attitude towards disposing
of the waste at the time. I hate to think where all that residue from
those Humble tanks that Shawn described went.....

Many tanks were not cleaned between trips for just that reason, and if the
stuff was hazmat they would be placcarded "empty" or "residue" for the
return trip.

Were covered hoppers similar dedicated? Can a man get inside a
covered hopper to clean out the previous load?

For the most part covered hoppers are dedicated to one commodity group.
And yes, they have access ladders so you can climb inside and clean it. In
the late steam era most covered hoppers were built for cement service, and
cement is hard to clean out of a car. After all, you don't want to use
water to clean it! So once in cement service a hopper tends to stay in
cement service, and they are rarely cleaned out.

Today "grain hoppers" are often used to haul fertilizer in the spring when
grain shipments are slow, and yes, they are cleaned out before hauling any
grain.

Scott Chatfield