Thank you for taking this trouble.
I agree that height clearance is to do with the trains being electric, but your explanation of the width clearance seems to be to do with speed of any passing train, not just electric trains. Your explanation makes perfect sense, but JL stated that electrics need more wriggle room, suggesting an additional factor at play besides speed and curvature.
There's a useful diagram in the Paisley Canal Low Cost Electrification PowerPoint presentation of platform clearance issues and OLE wire position.
http://nearyou.imeche.org/near-you/UK/Scottish-Region/Scottish-RD-Centre/past-presentations
You'll need Office PowerPoint, the PowerPoint viewer thingy, or OpenOffice etc to view the file.
The good thing is that generally you can pair the contact and catenary wire to pass under lower clearance obstacles (bridges etc) and increase contact wire height at the same time, as long as the line speed isn't too high.
The kinematic envelope thingy, just using the BREL Mark 3 MU family as an example - a Class 150 and a Class 319 have the same body, but there are a couple of subtle differences, the motor vehicles have different bogies, and all bogies are fitted with yaw dampers to control excess body movement at 100mph (BREL didn't think it was an issue at 75mph - I think they're wrong) so there may need to be a bit of additional spacing provided below the solebar level to accommodate this.
The kinematic envelope thing is perhaps best explained by thinking about going round a corner in a Transit van at 10mph, 30mph and 50mph, you know if you go round a corner at 10mph, it won't roll about much, it will roll a bit more at 30mph, and at 50mph, you'll think you're going to roll it. The same thing happens with a train, but it's fixed to the rails, so it leans and moves slightly (yawing) on the suspension, so the top of the coach moves further from the centre line of the rail, and the whole coach may move latterly against the direction of the curve. Higher speeds mean more energy, which mean more movement.
