With the southern end of the ECML about to get considerably busier does that mean the slow lines will be upgraded to allow trains to run faster and hopefully avoid the current pain caused by the 313 units and there pedestrian top speed and acceleration once the 717's arrive?
What is the current limit on the slows, up and down and what might they increase to?
From memory it's 60 mph inwards from Alexandra Palace, generally 75 mph to Hitchin, and 80 mph between Hitchin and Peterborough.
The 717s will no doubt give some level of performance increase as they should have better acceleration, but I wouldn't expect there to be much gained from any line speed improvements - the stations are simply too close together for much chance to exceed 75 mph, even with smart acceleration.
There would be some benefit from the slow lines being uprated to 90 or 100 mph north of Woolmer Green however, for the outer-suburban services. A 365 can quite happily get close to 100mph if routed on the down fast between Stevenage and Hitchin. Apart from track works this would require resignalling.
One constraint to the slow lines is bridge and platform alignments - there are quite a few places where the slow lines curve slightly. Brookmans Park and Knebworth are both examples of this - in the case of Brookmans Park IIRC there's a 55 mph PSR through both slow platforms.
