After months of wrangling and controversy, Transport Secretary Stephen Byers has given the green light to part-privatisation plans for London Underground.
But he is on a completely different track to many MPs and the capital's mayor, Ken Livingstone, who prefer a proposal to raise cash through bond issues.
So how do the two schemes differ?
Government's preferred option, under PPP London Underground (LU) would hive off Tube's infrastructure - track, tunnels, signals and stations - to three private "infracos" for 30 years
One would be responsible for Bakerloo, Central and Victoria lines; one for Jubilee, Northern and Piccadilly lines; one for the Circle, Hammersmith and City, District, Metropolitan and East London lines
Government would then hand control of Tube to Transport for London (TFL) which is accountable to the Mayor of London
Operation of trains would remain in public hands, under control of LU
Infracos would embark on £13bn modernisation programme over 15 years. Each would receive regular payments from LU but risk fines if they fail to meet targets
Finance would be divided between government subsidy (45% in first 7.5 years), LU revenues (30%) and private finance (25%)
Timetable for first 7.5 years would include 60 miles of track replaced and repaired; 500 refurbished or new carriages; 50 stations modernised; new signalling on Victoria and Northern lines; CCTV in trains and stations
Work would continue over following 7.5 years, but no details exist
Arguments for: Government has used private finance to fund building of new hospitals and prisons. Enables large sums of money to be raised without affecting government borrowing
Arguments against: TFL says might be difficult to enforce contracts if private companies opt to pay fine rather than undertake work
London Mayor Ken Livingstone's preferred option, under bonds scheme London Underground (LU) would pass to control of Transport for London (TFL) but remain as a single, publicly-owned entity under "united management control"
TFL would set about raising £12.75bn funding for modernisation over 15 years
Fund raising would be divided between fares revenue and government grant (64%), bond issues (34.3%), and rest through existing private finance initiatives
Priority improvements would focus on factors which cause "most system failures": rolling stock, signals and track
This would include mechanical refurbishment or replacement of 85% of rolling stock within 7.5 years; signal improvements at "pinch points" and track improvements where needed
The second stage of modernisation would deal with expanding Tube to cope with expected increase in passengers, and improvements to existing stations
Arguments for: As head of New York's Subway, Bob Kiley, now the boss of Transport for London, used bond issues to successfully improve the service. Bond issues have been used to raise money for the Channel Tunnel Rail Link
Arguments against: The government cites delays and other problems with the publicly-funded Jubilee Line extension as a reason for separating big programmes from public funding