The ref is the Network Rail engineers line reference (ELR) which refers to the rail line, not the bridge. Bridge references are formed of the ELR and an ordinal number (e.g. ECM1/204), and these are indicated by signs on the bridge.

The ref is Network Rail's Engineers Line Reference (ELR) for the line. These can be found on signs on bridges along the route e.g. "This is bridge GSM3 2 - High Street, Syston". Not exactly helpful for a rail passenger, but extremely helpful if you're a lorry driver who has just been involved in a bridge strike.

Edits reverted by changeset/36040444 - the Waikato Expressway is not a designated motorway.

I stand corrected - it should be passenger_lines=4 as far south as the old Clay Cross South Junction. I've even double checked against the sectional appendix and there appears to be a break in mileage and a change in engineers line reference at that point too. I'll correct it shortly.

You have to tag at least one way passenger_lines=2 for the renderer. Tagging the other way is just for redundancy (and I'm a bit OCD on ways matching).

"Ah, so you're tagging for the renderer!"
You did read the section entitled "clarification"? You're allowed to enter tags for a specific renderer to understand, as long as they're not completely misleading (e.g. tagging flowerbeds as industrial areas).

One closer to home (for me) are way/106857413 and way/106857420.They are labeled passenger_lines=2 despite there being four parallel tracks because the two central tracks belong to a separate rail route and are super-elevated relative to the outer lines. They do not physically join together until Wellington Distant Junction to the south near Aotea Quay. (node/3235330766)

Here is the rendering of passenger_lines: http://product.itoworld.com/map/231?lon=-1.65321&lat=53.42826&zoom=8.

So if a two-lane road merges with another two-lane road to form a four-lane road, I should stat tagging them lanes=4 as soon as they are parallel, not when they merge?
Standing on the Station Road bridge looking south you can see the two sets of lines diverging while looking north you can see there are no points or crossovers connecting them, meaning they're separate parallel lines at that point.
"a local mapper (like me) is supposed to be able to go to a place and say "yes, I can see that"." How do you know that the track gauge is 1435mm - have you actually risked life and limb to measure that? Or that the electrification is definitely 25,000 volts?
Nobody else has complained about the usage of the passenger_lines tag in the whole time I was tagging west of the Pennines - it's when I venture east of them that hostile mappers bite everyone and anyone.

There are four parallel tracks geographically, but two sets of two parallel tracks topographically until you reach the junction north of Bridge Street/Hagg Hill. Another example would be the Underground District and Piccadilly lines between Acton Town and Earls Court - they run parallel geographically but topographically they never connect and therefore are separate lines.
And did you even read the wiki article? It specifies renderings where passenger_lines shows.

There are only two parallel lines at that point (the third appears to be a loop). Once the Midland Main Line and Erewash Valley Line physically join closer to Grassmoor, it goes up to passenger_lines=4.
OSM can't detect multiple parallel tracks and when you're looking at zoom 12 or lower, you can't see individual tracks. Passenger_lines bridges this gap.

passenger_lines indicates the number of tracks a line has, e.g. passenger_lines=2 indicates a double track line. It replaces the deprecated tracks key. See passenger_lines=*

Sorry - tagging error on my part. It has been reverted to Erewash Valley Line

Ways deleted or shortened partially due to carelessness of reverter (list is incomplete)
way/3469285
way/292150815
way/367741232
way/367742752
way/367742755
way/367742820
way/367747455
way/367747458
way/367786116
way/367786117
way/367786118
way/367786120
way/367792699
way/367790099
way/367790100
way/367790101
way/367790102
way/367790103
way/367790104
way/367790105
way/367790106
way/367790107
way/367790622
way/367790623
way/367790624
way/367792702
way/367792703
way/367792706
way/367792707
way/367792708
way/367913826
way/367913827
way/367913829
way/367913830
way/367913832
way/367913834
way/367913835
way/367913836
way/367913837
way/367912262
way/367912265

Also:
way/3469285
way/292150815
way/367747455
way/367747458

Sorry, those were from changeset/33890248.

Here are 26 of them:
way/367741232
way/367742752
way/367742755
way/367742820
way/367786116
way/367786117
way/367786118
way/367786120
way/367792699
way/367790099
way/367790100
way/367790101
way/367790102
way/367790103
way/367790104
way/367790105
way/367790106
way/367790107
way/367790622
way/367790623
way/367790624
way/367792702
way/367792703
way/367792706
way/367792707
way/367792708

@SomeoneElse It wouldn't have happened if the reverter wasn't lazy.

This edit over-reverted and ended up removing a number of railway lines rather than just reverting speed limits to mph.

Based on Wikipedia article "Cross Country Route", but can also be called "Crewe to Derby Line" as far west as North Stafford Junction (near Willington)