I've done some of that stuff. One afternoon, I shoved
a freight train over Newcastle with my 'hopper' without
cutting off, or 'cutting in'. Like you say, it could have
been messy, but wasn't, he really didn't take much
shoving. I've had the Belle Ayr helps cut off on the fly
with the crew on the waycar, and no air cut in. Sometimes
they weren't even coupled, just pushing against closed
knuckles.
If you helped another train on the road, it paid time
or miles, whichever was greater, with a minimum of two
hours (25 miles). The time started when you were notified
by the Dispatcher that you would have to cut off to shove
them, and ended when your train was moving again. The
miles were counted from where you cut off to where you cut
off the train you were shoving, and back again.
In reference to the original question, I've always read
that with steam helpers, the helper would try to start the
train first, then the road engine would start pulling,
though Karl would probably be in a better position to
comment on that. The principle was that the Helper was
'bunching' up the slack, so that the Road engine didn't
have to start the whole train at once. The two Engineers
would communicate by whistle signals. The helper had the
automatic brake valve cut out, with what was commonly
called the 'double heading cock'.....though the
instructors at BNU in St. Paul would loudly inform you
that it was properly known as the 'brake pipe cut out
cock' :>)
Mike