London’s (surface) Rail network, illustrated below, is superbly engineered, often grade separated and often wide enough for dual 2 or 3-lane motorways. It overlays the road network, which is, for the most part, a collection of tarmacked access roads.
Half a million surface rail passengers enter central London in the three hours 7 am to 10 am (Transport Statistics Great Britain). Hence it is generous to say that 250,000 do so in the peak hour. There are at least 25 pairs of tracks. Hence, per inbound track we have an average of 10,000 passengers in the peak hour.
The 10,000 could all find seats in 200 50-seat coaches or in 150 75-seat coaches, sufficient to occupy between one seventh and one fifth of the capacity of one lane of a motor road (1) the same width as required by a train, see fact sheet 3
That is to say, even in the peak hour and in central London this immense rail network is used to between only one seventh and one fifth of the potential that would arise if the network were converted to a system of motor roads managed to avoid congestion.
Outside the peak periods the rail network is a place of dreams. In contrast the roads are clogged with traffic all day.
(1) Highway capacity manuals provide a capacity 1,500 vehicles per hour per lane. Chiswick flyover was carrying in excess of 2,000 per hour per lane in the late 1960s. Hence a capacity of 1,000 express coaches per hour per lane is realistic. At 60 mph (100 kph) those coaches would have headways averaging 100 metres. On UK motorways headways are routinely less than that.