The length of a train, including the longest trains, may be measured in number of wagons (for bulk loads such as coal and iron ore) or in metres for general freight. Train lengths and loads on electrified railways, especially lower voltage 3000 V DC and 1500 V DC, are limited by traction power considerations. Drawgear and couplings can be a limiting factor, tied in with curves, gradients and crossing loop lengths.
Conventional freight trains can average nearly 2,000 metres. Freight trains with a total length of three or four times that average are possible with the advent of distributed power units, or additional locomotive engines between or behind long chains of freight cars (referred to as a "consist"). These distributed power units enable much longer, heavier loads without the increased risks of derailing that stem from the stress of pulling very long chains of train-cars around curves.
Source: Wikipedia
