Posts Tagged ‘train’

How a steam loco differs from diesel-electric loco?

2013/09/23

A diesel-electric (DE) loco is a constant power machine. We know that, Power = Force x Velocity. This means, when hauling heavy loads (especially on an uphill), a DE loco can increase draw bar pull by reducing speed. This also shows why most shunting locos are DE locos as they need to haul heavy load but at low speed.

 

However, due to the way a steam loco is designed (i.e. its boiler, piston and other mechanisms), a steam loco behaves as a constant force machine up to cruising speed (usually around 25 MPH or 40 km/h). Beyond that, a steam loco behaves like a constant power machine like a DE loco.

 

This difference is crucial. For a steam loco to haul heavy load (or on an uphill), if it can’t deliver enough draw bar pull, it has no way to increase the force like a DE loco, because for steam loco, force is constant at low speed. So, a steam loco won’t be able to climb a slope like a DE loco. It will literally run out of puff under such circumstance. However, once a steam loco has crossed its cruising speed, it will have no problem pulling heavy trains as it behaves in same way as DE loco i.e. constant power loco.

 

There are obviously other differences between these two types of locos. A DE loco is more thermally efficient than a steam loco. The later requires far more maintenance than the former and needs more crews to operate them.

 

No wonder, due to these reasons, most of old steam locos have been replaced by DE locos in all but heritage routes. Please note that a pure electric loco will also behave similarly as of a DE loco.

 

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Why most US locomotives have single cab while European ones have dual cabs?

2012/12/17

Most US locomotives have single cab. This creates a┬ávisibility┬áproblem when operating long hood forward mode (visibility is usually ok while operating short hood forward mode). However, it is quite rare for US locos to operate solo – they mostly work in pair (sometimes upto 3-4 units together as well). As a result, they rarely run as long hood forward mode in real life.

But in Europe, the overall weights of trains are lighter and thus often trains are pulled by just single locomotive. Here both forward/reverse operation is desired. Thus European locos need good visibility in both directions (otherwise they would need turntables which are now obsolete after steam loco era).

Countries which imported American locos (like common WDM style locos in India, licensed from ALCO) but often runs as single units, visibility is indeed a problem while running in long hood forward mode.