This was originally written as a bit of a help page for @locospotr, a Twitter based train spotting site that was supposed to be a sister service to BeerSpotr and the sadly neglected CCTVSpotr. Unfortunately LocoSpotr never went live because I never quite finished the code and couldn't find a friendly designer to make the CSS.

So I forgot about this until Paul Carvill tweeted today:

from the Wall of my Facebook group for people on trains: "how about a QR code scanner that you can use to find out which train you're on?!"

Which made me think how cool QR Code trainspotting would be. Our duly elected (ish) leaders would no longer have to worry about impressionable british teenagers hunkering down over youpron. Instead they'd all be out sporting smart tanks tops and twonkPhones on the platforms of Clapham Junction eagerly snapping the QR Codes on the sides of trains.

All it would take is the various train operating companies giving all their locos (and units and classes) cool uris, some stylish Duncan Robertson QR Code stuff and a twonkPhone app. And we could save our nation's youth from death by decadence and wanking. Or maybe not. In the meantime:

Locomotive numbering

In order to identify and organise locomotives, railway companies usually give each one a number. These numbers are usually unique within the confines of the railway system and period. But they are not globally unique and not unique across time. Two locomotives on two different railway systems might share the same number. And a single locomotive might have many numbers over time. The Flying Scotsman, for example, has carried four numbers over its lifetime - 1472, renumbered 4472, renumbered 103, renumbered 60103.

UK locomotive numbering post 1973 - TOPS

In 1973 British Rail adopted the Total Operations Processing System (TOPS). This system gave each locomotive a unique number comprised of 5 (sometimes 6) digits. TOPS survived the breakup of British Rail and is still in use today. If you're in the UK and interested in locomotive spotting this is the number you want.

The trouble with 'multiple units'

Most modern passenger trains are comprised of either DMUs or EMUs. A multiple unit is basically a passenger train without a separate locomotive - every vehicle provides at least some passenger accommodation. They range from 2 coach local units which are little more than buses on rails to fast intercity units like the Virgin Pendolino.

Under TOPS, locomotives, carriages and units all have identifying numbers. Carriage numbering is much more confusing so I won't go into details here. In a locomotive hauled train there's little chance of confusion because the locomotive is obviously separate to the carriages. But in the case of multiple units the unit and all its individual carriages will have identifying numbers. Usually the carriage numbers are shown along the side of each carriage and the unit number is shown on either end. But the position of the unit number can vary between train operators.

For the purposes of LocoSpotr the interesting number is the unit number which is comprised of 6 digits (eg 444040). If you're interested in spotting carriages this probably isn't the site for you but if you spot a carriage that's particularly fascinating you can always add its number to your spot as a Twitter hashtag.

Finally multiple multiple units are often coupled together to form a single train. If you walk down a train and a see a driver cab area, that's the start of a new unit. Each unit has a number. The train as a whole doesn't - or not one you can easily find out. You might want to tweet a separate spot for each unit in the train - you might not.

A note on British Rail's HST - aka Intercity 125

Back in the 1970s British Rail introduced the Intercity 125 HST. Plenty are still running today. They're slightly unusual in that they're passenger units (so have carriage and unit numbers) but are hauled by 2 class 43 locomotives (one at each end). For the purposes of LocoSpotr the interesting numbers are the locomotive numbers (43xxx).

Locomotives with names

Some locomotives have names as well as numbers. Back in steam days the names were usually more persistent than the numbers (eg the Flying Scotsman changed its number 3 times but never changed its name). These days numbers tend to be more persistent than names (eg Pendolino 390010 was originally named Commonwealth Games 2002, renamed Chris Green, then A Decade of Progress).

It's probably better to identify the locomotive by its number than it's name. You can always add the name as a separate hashtag:

@locospotr loco:390010 class:390 #adecadeofprogress

Spotting preserved locomotives

These days there are lots of preserved locomotives running on heritage railways. The majority predate the TOPS system and many predate British Rail. In the days before BR, locomotive numbering was much more fiddly. Each railway company had its own numbering system and many of these systems overlapped. When UK railways were nationalised the locomotives BR inherited were renumbered (this happened a few times before TOPS was introduced).

Many preserved locomotives were either withdrawn before TOPS happened or have been restored to their pre-TOPS livery / numbering for the purposes of nostalgia. There's a lot of debate amongst blokes called Trevor about whether a locomotive that's been technically altered since it first carried a certain livery / number should carry that livery / number in preservation. Which we'll skip over for this intro.

The obvious question is, having spotted a preserved locomotive, which number should you record it under? For the purposes of LocoSpotr it's probably best to record it under the number it carried when you spotted it.

Non-UK locomotives

There's not much to say here cos:

  • I don't really know all that much about forun trains
  • I'm not sure that train spotting is an obsession that extends much outside of 1950s Britain


Locomotive classes

Having gone to the trouble of designing a locomotive it's very rare for only one example to be built. Usually many locomotives are built to the same design. The set of locomotives built to a single design is called a class. Often there'll be minor variations in design and build between locomotives in a class; these variations are usually referred to as a sub-class. But in general all locomotives belonging to a class will be recognisable as part of the same family. Think of locomotive classes like car models: there might be a diesel Ford Focus, a petrol Ford Focus, a 3-door Ford Focus, a 5-door Ford Focus but they're all recognisable as the same basic model.

Again railway companies usually give each class a number which is usually unique within the confines of the railway system and period. But again they are not globally unique and not unique across time. Two different classes on two different railway systems might share the same number. And occasionally class numbers are reused over time; there have been 2 completely different class 70s under the BR TOPS system for example. Tut.

UK class numbering post 1973 - TOPS

When British Rail adopted TOPS each locomotive class was assigned a unique number comprised of 2 or 3 digits. This system is still in use today. Diesel locomotives fall into classes 01-69, DC electric locomotives 70-79, AC electric locomotives 80-96, departmental locos (those not in revenue-earning use) 97, and steam locomotives 98. Diesel multiple units (DMUs) with mechanical or hydraulic transmission are classified 100-199, with electric transmission 200-299. Electric multiple units (EMUs) are given the subsequent classes; 300-399 are overhead AC units, while Southern Region DC third rail EMUs are 400-499, other DC EMUs 500-599.

Luckily for train spotters the TOPS class number is incorporated into the TOPS locomotive number as the leading 2 or 3 characters. To get the class number just take the locomotive number and remove the final 3 characters. So a locomotive with the number 66713 is a member of class 66, a locomotive with the number 390010 is a member of class 390 and a multiple unit with the number 444040 is a member of class 444.

To make things simpler still there's a useful pictorial guide to some of the common BR TOPS classes at Wikimedia Commons.

Classes with names and preserved locomotives

Some locomotive classes have names as well as numbers. Sometimes these are officially sanctioned, sometimes less so. Back in steam days names were often used in preference to numbers (e.g. King class, Castle class, Merchant Navy class).

For the purposes of LocoSpotr if the locomotive class has ever had a TOPS number (even if the locomotive hasn't) it's probably better to use that. You can always add the class name as a separate hashtag:

@locospotr loco:d1005 class:52 #western

If the locomotive class pre-dates TOPS and is known better by its name then use that:

@locospotr loco:35005 class:merchantnavy

Many locomotive classes that pre-date TOPS never earned anything but a nickname. Usually these classes were given alphanumeric labels that sometimes reflected their power classification, sometimes reflected their primary use (e.g. a class 9f was a powerful (9) freight (f) locomotive) and sometimes reflected nothing at all. In these cases just use that label:

@locospotr loco:4472 class:a3 #flyingscotsman