# How does train back draft work

Quoting platforms and stations :

Fast trains can create a vacuum called a ‘back draft’ that can blow you over or suck you under a train.

How does that work ? I thought that the volume of air displaced by the moving train would, on the contrary, push one away from the train.

• The mythbusters did investigate this too Commented Sep 26, 2014 at 14:16

The front of the train compresses air which can blow you away, while at the back of the train air rushes back in after the train has displaced it. This backdraft is especially troublesome in closed areas such as subways, where a train exits a small tunnel near a platform and the displaced air rushes back into the vacated tunnel. Next time you see a big truck rushing by watch what happens to some grass or plants near the side of the road, you might be able to see the backdraft.

• + And if the train is long, it carries air with it along the sides, lowering the pressure, so pulling in careless creatures. Commented Sep 26, 2014 at 13:42

Like any object moving through a fluid, a high-speed train distorts the air as it moves through it.

Broadly speaking, there are three main regions of flow structure around a high-speed train: the upstream distortion, boundary layer and wake. These can be collectively referred to as the slipstream. The effects of the slipstream on a static observer (e.g. a person on a platform) vary depending what part of the train is passing:

From the observer's perspective, air accelerates immediately before the nose passes, followed by a sudden deceleration (i.e. a pressure pulse, or gust).

### 2. Body Passage

After the initial gust, slipstream velocity begins to increase again as a boundary layer grows on the side wall of the train/carriages. This is going to be a relatively steady build-up but carriages, undercarriage etc. will impact this.

### 3. Tail Passage

Immediately after the tail passes the wake is already being generated. There is another big velocity spike here as the wake meets the observer. In addition, the exact structure of the wake depends on (train) geometry, but it may have:

• a separation bubble / recirculation region where the flow detaches from the train body.
• shedding vortices

Generally a wake is going to be lower-than-ambient pressure, but in the case of vortices, they can actually result in air being accelerated down behind the tail, creating a low pressure region and sucking air in.

## "back draft"

It's not a very descriptive term IMHO but anyway:

• it's possible a push-pull type of effect may occur in the situation where the is indeed a low pressure region behind the train actually sucking air in (the gust hits, then the suction).
• additionally, the closer you get to the train body the higher the slipstream velocity. Although this will produce a force in the direction of motion, if the slipstream sweeps an object up it may effectively get sucked in and behind

It's very much not about volume of air displaced, but rather how it's displaced.