GPS on Railway

Someone asked recently on Linkedin, why aren’t railways using GPS for their train positioning. Here’s my answer below:

1. GPS is limited by infrastructure and obstruction
2. GPS has lower accuracy, say for example, a norming point/balise. Quick google and saw it says around 1 – 5 meter accuracy.

Based on this two points, it’s not quite useful for
A) metro because most are underground or in crowded city with ‘weak’ GPS signal
B) mainline that has more than single track. Let’s say the mainline train loss comm, it needs to initiate its position again. How does it know whether it is on the North Bound or South Bound track? It might be able to move over a short controlled distance, but since the tracks are normally parallel, the train will still not ‘know’ which track it is on. A balise will solve that.
C) single line might be able to use GPS, as it doesn’t have the problem in B. But, single line working might even use simpler principle (single train working, token, track circuit)

That’s my opinion. However I think the main reason GPS is not so ‘popular’ for railway is because it is a relatively newer technology, in comparison to other train detection method, hence present cost of adoption. Train track are also identified clearly. Compare that to road user which has open path, GPS brings more benefit.

I think the question is quite good and I used to think about it before this. At that time though I didn’t quite understand how train control manages the location of the train (positioning). Glad to see the question as it reminded me about how we sometimes think things seem easy not because we know a lot about it and know how to do it perfectly, rather we are unaware of the complexity.

Hope I’ll avoid this in the future and hope the answer above is useful.

Thank you for reading.