Given that a sand drag is intended to stop a train, how does a train restart in one to return to the main line or not actually stopped who decides it has slowed enough to negotiate the exit (without a speedo)?
Answer to how does a train re-start... Carefully!!!
Apart from the fact that trains (and odd items of stock aren't actually supposed to end up in a drag - or in the dirt - if they do mess about there is likely to be a question of lining the wheels up with the rails. This can be done...
I have seen 3 axles of a Crompton in the dirt and restored to where it should be with the judicious application of fishplates - yes - this is possible... unfortunately they then put a couple of axles off at the other end
It is also possible to drag 24 grampus that had got bored with following the rails of a siding back on.
These operations are done
very slowly!
Facing Traps are very rare outside of locations where movement will be slow. In all my experience it is necessary to close the trap (i.e. set the route into continuous track) before a movement can be signalled toward the trap. On anything but very slow lines it is quite usual for there to be either a very long over-run between the last signal before the trap or for both the last signal and the signal on the approach to it to have to be cleared - with the facing trap set back into the continuous line.
This means that - apart from anything else - signals would have to be returned to danger/stop before the points could be moved. There is a significant chance that some form of release (e.g. a 2 minute delay or a wind-out) would hold the road after a signal or signals were put back. There is also a serious chance that the tracks would be track circuited and the route held by occupation of the tracks.
The result is that it is very unlikely that traps would be swung to derail a movement except where the movement would always be slow.
Facing movement runaways - especially at speed - would be a significant problem to divert... For a (major) start - if the movement is travelling fast (and for - short wheelbase - unfitted sand wagons 15mph is not slow) there would be serious issues of the movement potentially derailing through the points - even with a relatively low angle of departure.
Once into a trap - and particularly into a sand drag - there would be a whole load of questions to be answered before attempting to re-rail the movement... It would not trundle happily on from a far end as if it had merely been in a loop.
As for "blowing" to be diverted into a trap... This would require close proximity to the controlling Signalbox... No signalman is going to want to take responsibility for derailing a fast moving train any more than a footplate crew is going to opt to be dumped in the dirt.
The last said - One of my Inspectors did see a light loco try to sneak out against the signal as soon as he had set the route from a siding - and put the trap back - landing the loco (slowly) in the dirt... It was generally reckoned that no-one made Inspector until they had achieved such a feat or similar.