Why do some high speed trains like the Shinkansen E5 and E6, have really long noses while other trains, like the Eurostar and Javelin, have shorter noses?
How does the shape of the nose affect the train?
Engineering Stack Exchange is a question and answer site for professionals and students of engineering. It only takes a minute to sign up.Sign up to join this community
It all depends on the level of efficiency you seek.
A train, given its size, has a ridiculously small cross section. This small frontal area footprint is being 'pushed' by the inertia of hundreds of tonnes of metal.
All high speed train have aerodynamic noses, but some will tend to be pedantic about how many thousandths of a percent efficiency they can squeeze out of an aerodynamic profile.
There are a couple of other factors that come into play with the design of the nose and that is the pressure pulse that is generated as a train passes another object, generally another train going in the opposite direction or a tunnel. It's why passengers are told to stay away from the platform edge as fast trains pass by, its not that you'll be blown away, but may sucked towards the train by the low pressure.
Two high speed trains passing each other can generate a huge amount of suction between them at the nose, because of the low pressures there, so having a longer nose helps spread this out a bit and reduce the area of minimum pressure. If not the trains could be pulled towards each other, or some instability set up by the rapid sideways pressure pulse. The nose of the E5 Series, at 15 metres, is a massive 9 metres longer than the previous incarnation of the bullet train (Shinkansen), the E2 Series. This, according to its designers at JR East, will help eliminate the phenomena of "tunnel boom".
Japan's rail tunnels are somewhat narrower than their European counterparts, so when the Shinkansen enters a tunnel at speeds above 200 kilometres per hour, the sudden increase in air pressure can cause a loud "boom" at the other end of the tunnel. In some cases, such shock waves are thought to have damaged tunnels in Japan, ripping chunks of material from tunnel ceilings.
The shape of the front car has evolved gradually to combat this danger, and the striking "Long Nose" design of the E5 Series is the result.
In high speeds the impact of aerodynamic forces need to be included in analysis. Recall from the formula for aerodynamics
$$F_a = 1/2 \rho v^2 C_d A$$
Where $\rho , v, C_d,A$ are air density, speed of the vehicle , drag coefficient and contact area with air. The drag coeffient has other components. The viscous effect has an impact on the aerodynamics forces.