But arrived to this point, it emerged the desire of improving the traction, being adopted the one available then: mechanical traction by steam. And as usual, it was England which made the first experiments, but not with small locomotives built on purpose, but with steam engines installed in small carriages which towed the cars or in the cars themselves. Despite far from perfect, steam tramcars had a great development, to the point that in 1895 existed in New York about 5000 cars with a railway length of 1360 kilometers.
One of the first models of American tramcars.
A project by James Swett from 1853 for an elevated line. Curiously, the locomotive should not tow the passenger car, but carry it by hanging.
Existed as well experiments with traction by compressed air; in 1827 British engineer George Medhurst wrote an essay proposing such method for transporting passengers. The system consisted of a small car which would be literally suctioned by a pneumatic pump along a duct. In 1830 another British engineer, Samuel Clegg, solved the various inconvenients derived from the necessity of keeping the duct hermetically closed, by adding a longitudinal valve. This allowed to establish in the outskirts of London a section of which is called "atmospheric railway".
In France had success as well these "atmospheric" lines, despite the inconveniences: the machines required to move the vehicles had a considerable size and they had to be kept in operation constantly, despite the travels being quite unfrequent, with the consequent high cost in maintenance. However, this locomotion system found an economic utilization for transporting mail, from the offices in the periphery to the central offices, through underground tubes. It seems that the speed achieved with this system induced to establish underground lines for transporting passengers as well; the first attempt of this kind was effectuated in New York in 1872.
An metropolitan subway of circular cross-section experimented in New York in 1872.
But it was the adoption of electric traction in 1883 the true progress in the field of metropolitan public transport; in New York the line "Elevated" was provided with a third rail for supplying the energy to the tramcars. But the definitive system would be the one introduced by Frank Julian Sprague in 1885, who suppressed the electrified rail which produced shocks in pedestrians and animals alike, replacing it by overhead lines and trolleys which took the electricity from them. Milan was the first Italian city in which tramcars were installed, in 1893.
The subway transport was inaugurated in Milan in November 1964. In 2015, with a total length of 92 kilometers, this network has more than doubled the length of its counterpart in Rome.
Circa 1975 the metropolitan subways had these lengths (in kilometers) in the following cities: New York (381); London (348); Chicago (258.6); Paris (189); Berlin (94.1); Moscow (77); Hamburg (74.3); Tokyo (59.9); Mexico (42.2); Philadelphia (42); Stockholm (40.4); Madrid (39.5); Boston (37.3); Buenos Aires (30.7); Vienna (26.7); Athens (25.6); Osaka (19.9); Barcelona (17.7); Leningrad (14.5); Rome (11); Glasgow (10.5); Toronto (10.4); Lisbon (7.3); Rotterdam (6); Kiev (5.9); Budapest (3.7).
Circa 2015 the same metropolitan subways had these lengths: London (408); New York (336); Tokyo (338.7); Madrid (324); Moscow (317.5); Mexico (225.9); Paris (219.9); Chicago (170.8); Berlin (151.7); Osaka (129.9); Barcelona (123); Saint Petersburg (113.2); Stockholm (105.7); Hamburg (104); Athens (83.3); Vienna (78.4); Rotterdam (78.3); Toronto (68.3); Kiev (67.6); Philadelphia (63.5); Buenos Aires (58.8); Boston (51); Lisbon (45.5); Rome (40.4); Budapest (31.4); Glasgow (10.4).