Why are the front of commuter trains flat?

685 views

I’ve lived in or near 3 different cities (DC, Philly, and Trenton) and all of their local metro trains have had flat fronts. Wouldn’t it make more sense for them to be shaped more like cars or Amtrak trains when it comes to aerodynamics? I don’t understand how the flat front design could be efficient.

In: Engineering

You’re right: it’s not an efficient design for high speeds. And while it seems like your local subway or commuter rail might be going fast, there are practical speed limitations that normally prevent these trains from getting up to, or staying at a high speed where aerodynamics would really matter. The main one being, the stations are close enough together that the train barely has time to get up to speed before its starting to brake for the next.

A secondary factor: local trains are frequently electrically powered. Regional trains like Amtrack are not. So the impetuous to be fuel efficient and thus aerodynamic only is a factor for gas-powered trains.

The trains don’t move fast enough for it to be worth it when you’re considering other things like other drag factors and space efficiency

After you factor in all the weight of the entire train, and the long snake like shape, and the lower speed, i don’t think there is much savings at all. The cost savings of having similar shaped cars (and manifacturing/maintenance savings) probably outweighs the energy savings of having a different shape.

There are 2 main kinds of air resistance or drag at low speeds: form drag and skin drag. Form drag refers to drag resulting from a objects geometry on the front/leading or end/trailing edges while skin drag is caused by friction with the air along the objects length as air moves over its surfaces.

It turns out that for aspect ratios greater than 100:1 (an object 100 times longer than it is wide) that the skin drag will always be more significant that the form drag, so no matter what the front or end of the train looks like, the shape along the sides matters more. Thus if you are going to sink time and effort into reducing one, skin drag is the best bang for your buck and the flat front really isn’t that bad. This doesn’t mean a pointed end isn’t helpful though, especially as the average speed of the train increases.

There is the practical standpoint of maximizing interior face. A box has more room than an oval.

Commuter lines also need to worry about hitting people or running over them. If a person is on the track and the train hits them, a flat surface will do less harm than a point and is less likely for someone to get sucked under. Cars in recent times have modified bumper standards that prevent [too pointed of a front end](https://photos7.motorcar.com/used-1967-chevrolet-corvette-427435-8031-17585619-1-1024.jpg). Now they need to be like [cow catchers](https://cars.usnews.com/static/images/Auto/izmo/i67064107/2019_chevrolet_corvette_angularfront.jpg).

The flat face, like a school bus, can allow the driver to see anything in front.