I am afraid I am definitely against this.
TfL» needs to understand that its Elizabeth Line is just one user of one part of the
GWML▸ . It will not even be the sole user of the relief/slow lines - from what I have read outside peak hours it will share track with stopping
GWR▸ trains between Reading and Paddington. The line is the main artery of the rail system to a large part of the west of Britain, not a branch of London's suburban railway network.
TfL's network is mostly single user lines with tube trains playing follow my leader up and down or round and round (almost!) their route. The GWML has to combine 125 mph expresses for Wales and the west, semi-fasts and stopping trains for the Thames and Kennet Valleys and beyond into the Cotswolds, long heavy mineral trains for Acton, container trains crossing London, and all controlled from a signalling centre in Didcot. What experience has TfL in running an operation like this?
It has also not exactly covered itself in glory with its oversight of Crossrail either.
As to-
If done correctly, then both services and infrastructure could end up being overseen by a body that is genuinely accountable to both passengers and taxpayers, in a way that Network Rail has consistently shown time and again it has absolutely no interest in being.
How will TfL be accountable to the voters of South Bucks, Oxfordshire, Berkshire, Wiltshire, Gloucestershire, Somerset, Bristol, Devon, Cornwall, Worcestershire, Herefordshire and South Wales, none of whom will have a vote in the London mayoral election? Having a vital part of our rail infrastructure under the political control of someone answerable to an electorate who have little knowledge of, or interest in, our transport needs puts us in a worse position than we are in now.