A recent thread in the locomotives forum concerning double-heading 
of locomotives seems to have generated enough favorable response to 
my post (four people indicated that they got something out of it, and
no one said I was full of ----), that I thought I'd edit it a bit and 
add it to the library.  I hope you can get something out of it, too.
                                                  -JLS

>>  Seems like it would be real tough to sync them together.  I assume 
that each cab was manned - did they [have] any communications between 
them? <<

"Double heading" steam locomotives was very common throughout the 
steam era.  Also, helper locomotives were often cut into the train, 
maybe halfway or 2/3 the way back, and sometimes a helper was added 
as a pusher behind or immediately ahead of the caboose.  

(One of the most memorable pictures I've ever seen was published 
many years ago in Trains Magazine, obviously taken inside a caboose 
looking out the rear door at the unmistakable face of a N&W Y6b.  
Standing on this caboose, the only thoughts that would be in my mind 
is that this caboose I'm standing on is sandwiched between 10,000 or 
so tons of coal and 152,206 lbs of tractive force; and somebody 
doesn't think that coal is moving fast enough!)

The whistle and the brake pipe were the only communication between 
the engineers of each engine, unless they were close enough together 
to see hand signals.  The whistle was effective only as a starting 
signal, after that the engineers back in the train could not have 
heard the lead engines whistle due to the noise in their own cab.

If engines were double headed over an entire subdivision, the road 
engine (used to identify the train on orders, and usually with the 
senior engineer) was the lead engine.  If the double heading movement 
was only being made over a "helper district", the helper was usually 
the lead engine, to save time in coupling and uncoupling the helper.  
Some roads permitted the senior engineer to choose which position he 
wanted his engine, in which case he'd take the lead if there were 
tunnels in the district (or stay in the second position if he wanted 
to save time and hope for an "early quit").

The automatic brake stand on all engines except the lead engine was 
cut out.  All engines, including todays diesels, are equipped with 
a valve for this purpose.  The brakes on the entire train were 
controlled by the lead engineer.

On the whistle signal from the lead engineer to start, each engineer 
opened the throttle of his engine, and hoped.  Train handling under 
these conditions was extremely rough. If the lead engineer started 
too soon, he could "pull a drawbar" (Literally pull the coupler and 
all it's mountings right out of the end of the frame of a car).  
If the helpers back in the train started too quickly, they could 
actually squeeze the front portion of the train so hard that a car  
could be forced off even straight track.  Once moving, synchronizing 
the engines speeds isn't a problem.  Each engineer just worked his 
engine as hard as possible without slipping the drivers.

Engineers, other than the leading engineer, just "sensed" the trains 
movements to figure out what the lead engineer expected them to do.  
Watching the brake pipe pressure told them when a brake application 
was being made, in which case he'd reduce or shut off the throttle, 
and probably release the independent brakes on his engine so as to 
avoid overheating his driver tires.  If he felt the slack run out 
again, he'd assume more power was needed, and he'd open his throttle 
again.

Though no longer allowed by the rules, helper engines at the rear 
were sometimes added without cutting in the automatic brakes on the 
helper to the rest of the train.  Then, at the top of the helper 
district, the rear brakeman on the caboose would step out on the 
back platform, pull the chain to uncouple the caboose from the 
helper engine, and wave bye-bye.  The helper would thus be cut off 
on the fly.

For more information on the operation of a steam engine, download 
"Cab Ride Described", filename NP328.TXT in Library 2 Railfans.

>> I also assume that today, only the front loco is manned and 
all the others slave off it electronically, no? <<

Mostly, yes.  But not always.

When units are run in multiple, the engineer is in the lead engine, 
and controls direction, throttle, sanders, transition, air braking, 
dynamic brakes, etc., electrically and/or pneumatically on all units 
in the consist.  In some cases, mid-train slaves are controlled by 
radio.

However, some helper districts add a separately manned consist, 
either at the front, the rear, or partway back.  Operation of 
these units is similiar to the operation described above, except 
that radio communication between the engineers makes for smoother 
train handling. 

I hope this gives you some modelling ideas for your HO.

E:-)    -JLS