A spring switch and a non derailing switch are two totally different things. The spring switch points always remain in the same position allowing a train to run through when entering from the frog end of the switch, and then returns to the original position.
The non derailing feature switch throws the switch points over and leaves it there.
It does sound like in this case they are wanting a return loop on each end of a long straight track,. If that is the case, non derailing switch would work perfectly.
