Some of these might be realistic, others more suitable for this thread.
Four track the Crossrail and Thameslink cores, build a direct link between Tottenham Court Road and St. Pancras Thameslink, and add in services from King's Lynn, Southend, Oxford etc.
Four-track the Circle line (excluding to Hammersmith) but skipping 'minor' stations (e.g. no Great Portland Street and Barbican but have interchanges at Euston Square and Fenchurch Street).
Get East West Rail fully built, including Aylesbury, and fully electrify all Chiltern routes.
Electrify
- the Snow Hill lines
- London Paddington and Birmingham New Street to Carmarthen and Penzance
- Oxford and Birmingham to Hereford and Shrewsbury
- Birmingham to Bournemouth (overhead)
- Marshlink and Uckfield
- Cardiff to Crewe and Chester
- Crewe to Llandudno and Holyhead
- Kettering and Birmingham to Nottingham, Wakefield and Doncaster
- Manchester to York and Hull
- Doncaster and York to Hull
- Northallerton to Redcar
- Glasgow Queen Street and Edinburgh to at least Dundee
- the Harrogate, Saltburn, Exmouth lines... Any relatively short ones with at least 2 tph
- the entirety of Northern Ireland's network (if Newry to Dublin can be done at the same time so much the better)
Ensure that Northampton to Stafford via Birmingham is four-track, build extra platforms for local services at New Street and ensure that Northampton, Rugby, Coventry, Birmingham International, Wolverhampton and Stafford all have at least four stopping and four fast services to Birmingham per hour.
Double-track the entirety of Leamington Spa - Nuneaton and Oxford – Hereford.
Ensure that at least 4 tph can run between Cardiff and Swansea and between Cardiff and Bristol.
Trans or metros for every urban area with at least 200,000 people.
Get HS2, including the Golborne link, to the East Midlands, Sheffield, Leeds and York, built, and keep going south to Bristol and Cardiff.
Build a high-speed line from Liverpool to Hull / York via Warrington, Manchester, Huddersfield and Leeds with a branch via Bradford, and a new line from Doncaster to Liverpool via Sheffield, Manchester Airport, Manchester and Liverpool Airport.
Build a high-speed line between Edinburgh, Glasgow and Stirling.
Build lines to every town of at least 20,000 people.
Ensure that every line where it is not currently the case (e.g. West Highland, Heart of Wales, Esk Valley) has at least one train per two hours.