Comment by gmjoe | Hacker News Reader

Comment by gmjoe | Hacker News Reader

At the very end, about a current implementation:

> The free piston is magnetically coupled to the passenger modules above; this arrangement allows the power tube to be closed, avoiding leakage. The transportation unit operates above the power tube on a pair of parallel steel rails. The company currently has a 1/6 scale pilot model operating on an outdoor test guideway... The Corporation claims that a full-scale implementation would be capable of speeds in excess of 200 mph (322 km/h).

It sounds like magnetic coupling is the solution to the maintenance issues.

But it also seems like you can only have one train on a track at any given time.

Or, at least a track would have to be divided into sections, each with its own pumps and own piston, which could only support one train at a time... and then the piston would have to be sent backwards to the beginning of the section to be ready for the next train. (And you'd need electromagnets to let go of one piston and grab the one on the next section.)

So I can see why this might not be viable for something like a city's subway system.

But at the end of the day... what advantages would this ever have over electric trains that get their power from a third rail?

> can only have one train on a track

If desired, you could solve this by having a piston that can open up. Stick a butterfly valve[1] in the middle of it.

It might even be a significant advantage! A limiting factor of railway throughput is minimum headway[2]. You can't have trains crashing into each other, and trains are bad at stopping, so you need lots and lots of space between them. But if two trains are on the same tube together, they are naturally going to maintain a fixed distance between them.

Unfortunately, it's a bit tricky to take advantage of that because when they're close together, if you stop one, you have to stop them all. Also they all have to travel at the same speed, which is annoying if one needs to slow down for a turn.

[1]: https://en.wikipedia.org/wiki/Butterfly_valve

[2]: https://en.wikipedia.org/wiki/Headway

> But at the end of the day... what advantages would this ever have over electric trains that get their power from a third rail?

No heavy electric motors and other equipment in the vehicle, for starters

From the article:

> Fully loaded vehicles have a ratio of payload to dead-weight of about 1:1, which is up to three times better than conventional alternatives

Might make sense for a small system.

But electric machines have made huge advances in the last decades. Frequency converters, DC-DC converters, high torque permanent magnet motors etc are now very commonplace.

For example in the workshop, electric tools have supplanted pressurized air ones.

The advantage is the direct transfer of mechanical power. This is why many elevators today still use cables rather than putting a giant electric motor inside the elevator.

So why would a piston train be better than, say, a cable car?

The only real advantage I could see over a cable car is that the piston might be much efficient over long distances or winding tracks. Kinematic bending and stretching of a cable will produce heat which will cause a reduction in efficiency whereas slow changes in pressure tend to be isothermal.

"One apparent advantage of the cable car is its relative energy efficiency. This is due to the economy of centrally located power stations, and the ability of descending cars to transfer energy to ascending cars. However, this advantage is totally negated by the relatively large energy consumption required to simply move the cable over and under the numerous guide rollers and around the many sheaves. Approximately 95% of the tractive effort in the San Francisco system is expended in simply moving the four cables at 9.5 miles per hour" https://en.wikipedia.org/wiki/Cable_car_(railway)

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