I caught the Connex service from Northampton (only as far as Clapham J) on the first day, and my recollection was that originally it originated at Northampton, not Rugby - but I'm far from sure. Certainly the crew were very friendly towards passengers, a bit of a novelty for us.
The aim of Connex was, I was told by colleagues at the time, to reach Birmingham International, and that the reason was ORCATS-raiding more than service providing.
One thing the new service did require was route clearance of the literal sort. Because the 319s (for obvious reasons) had collector shoes, they were vulnerable to damage from rails, ballast etc left on sleeper ends, and the civil engineers had to put some time into checking this. Even so, later on a Rugby starter was sent old line because of a problem on the new line, and had its shoes removed by a ballast shoulder somewhere near Weedon. I think.
There was also a plan for a Great Eastern service to Northampton via Canonbury but this was probably just kite-flying, and possibly also motivated by ORCATS.