I think Rich's suggestion (see last entry) is a likely explanation for the loop, see the photo below which shows wagons standing there, which appear to be loco-less and brakevanless at one end. Of course they could be being shunted, which would still reinforce Rich's point.

There were few passenger trains and none at all after 1928; while Desford Junction, a couple of miles up the line would offer far better passing opportunities - with signalling too!

The coal trains from the Swannington end might also have been a little long for the loop at Glenfield.

Here are a couple of photos of this location:

First, that photo showing the wagons in the lop:

Secondly, a 2F in later days leaving Glenfield tunnel...

Chris