I was reading about the infinity train recently. This is a battery powered electric train that never needs recharging, at least not in normal operation. This sounds impossible, but in this case it works because the train is used in mining operations. The minerals are mined in the middle of Australia and then shipped to the coast; the point being that energy regenerated from the braking on the way to the coast, when the train is fully loaded, is used to charge the batteries to return the train to the mine when it is empty. It’s like a virtual counter-weight system.

Of course, most of this is not new. Railways have used regenerative braking for a good while. The use of batteries, however, means that the train line does not need to be electrified which makes it a lot cheaper.

I started to think about a generalisation of this technique though, because it only works when carrying heavy goods downhill. The desire is to use only electric trains on a line, but without paying the cost of electrifying it, while minimising the size of the batteries.

Well, even if a train does not have power available along the whole of its track, it is likely to cross grid connections periodically. Or, if the train is based in Australia and there is a lot of outback, it will pass a lot of land where renewable energy could be generated either from wind or solar.

If this is the case, then we could be build what I am calling a Slow Flow battery train. The idea is that the train would carry some battery carriages at the back of the train. When passing an grid connection or renewable resource, it could simply decouple empty batteries carriages and pick up some fully charged ones. Of course, stopping a big train is a time consuming business, so to make it work best this would have to be done autonomously and on the move. The empty batteries would then be parked at the stop and slowly recharged, waiting for the next train. In essence, the batteries would move along the train line, just at a slower rate than the train itself.

Of course, you can think of other topologies. Instead of having the batteries move the full length of the track, they could shuttle up and down small sections of it. This would allow all sorts of cleverness. The train could use more batteries where the gaps are longer, less when the next battery stop was available; where the train needs a lot of power, such as on an incline, Lithium Ion batteries could be used, while on flatter territory, these might be switched out for vanadium flow.

It might also be possible to dual purpose the train line. Imagine one of these battery charging stops which was associated with a non-grid connected renewable resource. Sometimes, trains using the line might take all the power; but at others there might be an excess. A passing train might pick up a few extra batteries and take them onward to a local grid connected stop where they could discharge in support of that local grid. How much energy this would cost and how that would compare to long distance transmission loses, I don’t know; but for sure the capital costs of adding extra batteries onto an existing train line would surely be less than stringing up a new electricity line.