As long as no other trains are coming!

If you are just dropping a rake at the start of the branch, and then return to the rest of your train, and there were no other trains expected - I see no reason why it would not work.

P.S. Taking the route of "simplest and easiest solution" is also the most "Railway" in that a railway attempts to do its job in the most efficient (uncomplicated) way possible.

"whether it's acceptable

    Since there's only one mainline track there they'd have little choice but to leave the train there for a few minutes while they set out the cut for the branch spur. They could just pick a time when no passenger trains are due , make the set out and be gone in a few minutes. If it was a busier area they'd probably add a passing siding so freights could get out of the way of passenger trains......DaveB 

Leaving Cars on the Main

If I follow you correctly, you're basically asking if part of a train can be left on the main track while the engines go in and switch some industries on the spur/branch. Short answer is yes, happens all the time.

Whether that train needs to concern itself with other traffic on the line while performing that switching is a much longer answer that depends on the methods of traffic authority and control in use...

Follow the prototype

I am not a railroader, and am much more comfortable 'lifting' track arrangements from working railroads than cooking them up in my brain. In other words, if a real railroad successfully operated trains with a given track arrangement, it would be good to adopt it for our models as well.

That being said, the scenario you describe probably reflects the reality at thousands of small station stops. Leaving the train on the main is a long-practiced necessity, as the great responses above mention. A passing siding is always a good resource for crews at station locations, but they are not always present.

How prototypes used facing and trailing switches, sidings, etc. is another question. Here, necessity again dictated the railroad's approach. Simple sidings can be added with little risk, but designing locomotive service facilities, yard ladders, junctions and other more complex track arrangements are better left to the real railroaders. I would rather copy what they do (even if I don't fully understand the reasoning) than wing it on my own limited experience.

Just my two cents...

Thanks all

Thanks all, this is useful. It may be a case of adding a passing siding into the mix somewhere, probably along the mainline. In fact, there is already a passing siding on the loop, back west of the bit I've drawn, that could be used for this purpose if brought closer to this point on the line.

@Chris van der Heide:

I've cooked up a bit of a homebrew idea for what's going on with traffic control, namely that the entire trackage visible on my layout is 'in yard limits' i.e. trains are driving very slowly under control of a dispatcher sitting in a signal tower. But to be honest: I don't know enough about traffic control. 

@Verne: 

I agree. The situation round my station is something that's rather been forced upon me by space considerations + wanting to have a loop + wanting switching and passenger ops in the same location. The switching areas beyond all bear some kind of relation to something I've at least seen a picture of.  

Just to clarify about Yard

Just to clarify about Yard Limits.

No person controls train movement in Yard Limits, which, BTW only concerns movement on the main track (not yard or industry tracks) within Yard Limits. Within Yard Limits (locations of which are shown in an employee timetable and signs along the track) trains and engines must operate at Restricted Speed which allows them to stop before hitting anybody or anything but not exceeding 20 MPH. This means a hundred trains could all occupy and move (if there was room) on the main track within Yard Limits as long as they could safely, stop before hitting a person or train but never exceeding 20 MPH. This is the gist of Yard Limits...there are also many other details covering its use.

On a switching, model railroad which is primarily a slow speed road, Yard Limits granting authority over the entire line is a good choice.

Barry

Clarity

Barry, thanks for explaining that, I think I'm beginning to see the light. Is there any general set of rules of thumb for how trains operate once they're off the main and into industry sidings? 

Rules for Other Than Main Track

for most railroads when operating on Other Than Main Track they are operating at 10 MPH and must stop short of Trains, Engines, Rail Cars, Men or Equipment foiling the track, Stop Signals, Derails or switches lined improperly. 

In in the 40's and 50's they would most likely be using Timetable and Train Order. It would be the responsibility of the inferior train by class of direction to clear the track and not block the passenger train. So if they could come into town do their work and get out without interfering with another superior train it would be no problem.