The length of a train 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 and power considerations. Drawgear and couplings can also be a limiting factor, along with curves, gradients and crossing loop lengths.
Very long freight trains with a total length of 3 km (1.9 mi) or more are possible with the advent of distributed power, or additional locomotive units between or behind long chains of freight cars (referred to as a "consist"). Additional locomotive 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.