[Tagging] Feature Proposal - RFC - 3rd and 4th rail

Colin Smale colin.smale at xs4all.nl
Tue Jun 9 21:57:38 UTC 2020

When I just checked around Gunnersbury I noticed that someone is already
retagging the London Underground to electrified=4th_rail so this
discussion is probably already irrelevant.... 

On 2020-06-09 23:12, Michael Reichert wrote:

> Hi Colin,
> Am 09/06/2020 um 15.36 schrieb Colin Smale: 
>> Great idea. Not sure about using "3rd" and "4th" though - it's a bit
>> tightly coupled to the English language and possibly prone to error.
>> Wouldn't "3rail" and "4rail" fit the bill? 
>> Actually, as electrified=rail is so widely used at present, how about
>> making that explicitly "3rd rail" and introducing a new value for the
>> 4-rail system?
> I am in favour of splitting "rail" into two new values for systems with
> a 3rd and a 4th rail (no matter how it is spelled exactly in the value).
> Currently all 3rd rail and 4th rail systems are tagged as "rail", aren't
> they? If we followed your suggestion, "rail" would mean "3rd rail or 4th
> rail system" and "4th rail" would mean "guaranteed 4th rail system".
> It's a bit like "yes" is a incomplete value for electrified=* which
> should be replaced by a more precise value like contact_line if one
> knows it.

It would have the heuristic advantage of already being right in almost
all cases, thus minimising the re-tagging effort. 4-rail systems are
quite rare actually, and well-localised. 

>> How would we indicate the voltage/frequency of the two rails
>> independently? On the London Underground it's mostly +420/-210 (these
>> days +500/-250) but there are some areas where +750/0 is used
>> (Richmond-Gunnersbury for example).
> Is voltage=* used on that lines as the difference between positive and
> negative, i.e. voltage = 750 = 500 - (-250)?

Yes it is at present, giving no way of distinguishing between +500/-250
and +750/0. The reason for using +750/0 is that the track is shared
between 3-rail and 4-rail trains, both operating at 750V. Normal
"Underground" track electrified at +500/-250 would only give 500V to
these trains, and the running rails are not intended to carry the return
current. The track signalling would probably get upset. So this
distinction is necessary to show (in)compatibility between sorts of

An obvious (but probably controversial) candidate would be
voltage=500;-250 - How about voltage:outer=500, voltage:centre=-250? 

>> While we are at it, could we take the opportunity to find a way to
>> represent three-phase dual-overhead systems (Switzerland etc) as well?
> For readers being confused: Gornergratbahn in Zermatt uses it.
> https://en.wikipedia.org/wiki/Gornergrat_Railway

And indeed the Jungfrau. It is currently tagged as voltage=1125,
frequency=50, electrified=contact_line which is correct but incomplete. 

We are missing two characteristics here. Firstly that there are two
overhead wires instead of one, and secondly that the power supply is
3-phase instead of single-phase. 

Might I also mention that we don't have a "clean" way of tagging for
dual electrification? I know of plenty of cases of 3rd-rail DC combined
with a overhead line, sometimes for transitioning from one system to the
other, and sometimes to accommodate different stock on the same lines
such as heavy rail on overhead power and metro on 3rd rail.
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