There seem to be several big questions about the SMJ.
Why was this built and to what extent was it ever used?
It seems that there was a chord, there is evidence for it. It seems also that there was a bay platform constructed at Roade station, but there was no physical rail connection with the LNWR.
If the passenger service from Towcester to Olney only lasted intermittently for 4 months what about the Roade service. Would the “main line” services have reversed in and out of Roade?
And what real purpose did it serve, bearing in mind that there were much better connections with the LNWR at Blisworth. (I appreciate that the S T & M J and N & B were different companies at the time that the Roade spur was built). I understand there was a lime quarry at Roade which may have provided traffic
According to BarryTaylor in his excellent pair of books on the SMJ the bay platform at Roade although fully signalled from the SMJ cabin at the top of the chord line was never used by any passenger train due to early withdrawal of the passenger service. Its possible that an occasional inspection train may have used it though.