The situation within the mainland Great Britain railway network is complex. Until the railways were privatised, the different regions of British Rail (BR) operated semi-independently. Often continuing using practices and design principles carried over from before nationalisation. And hence buying interlocking systems of their choosing or creating their own interlocking systems.
Hence some route relay interlocking systems came from signalling equipment companies and others were built by a region's own design and installation staff. For example, BR Western Region used their own design of "free-wired" route relay interlocking based mainly on relays to the BR930 series of specifications (one supplier, Westinghouse Signals called these Q style). P style were also sometimes used. The BR Western Region design of "free-wired" route relay interlocking is documented by "standard" drawings in the E10000 series of drawings, hence it is sometimes referred to as the E10K type. A web site with some more information on this is here:
Swindon Panel - Interlockings. Note that this site manly covers the area of control of the former Swindon PSB. But this was typical of the other BR WR PSBs (if one of the smaller PSBs).
A freedom of information request to Network Rail is here:
PDF listing interlockings used across the network
BR also developed it's own free-wired interlocking design.
A link to a current BR930 relay supplier is here:
Mors Smitt.
The RSSB has some BR930 specification documents (e.g. at
this link).
Some large route relay interlocking schemes were designed and built over the years. For example, during the 1960s through to the 1980s, BR Western Region built various power signal boxes (PSB) with NX panels. "NX" (Entrance-eXit) meaning, turn a switch or push a button to define the start of the route, then push a second button to define the end of the wanted route for each signal section. More details in the PDF at
this link. These PSBs covered large parts of the region, hundreds of route miles. Gloucester is an example of one of these that is still in use. If you search this forum, there is a topic on here about the rewiring of Gloucester PSB interlocking.
However, some parts of signalling were developed as national systems. BR developed a computer based interlocking called
SSI. SSI standing for Solid State Interlocking. Since the first practical conventional installation at Leamington Spa, SSI has been used all across the country. Although mostly designed by BR, there were two suppliers of production equipment: GEC-General Signal and Westinghouse Signals Ltd. The equipment from both of these could be mainly mixed with no problems. So you may find some from each supplier in a practical scheme.
SSI can be used with conventional switch / button / indication "NX" panels or other panel designs. And some examples of this exist and are In still In operation today.
But BR also developed a computer based control system. Called
Integrated Electronic Control Centre (IECC). This was designed to connect to and control one or more SSI systems.
Since then various systems have been developed and used, including Smartlock and Westlock that MarkyT mentioned.
In terms of suppliers, during BR, there were many signalling equipment manufacturers. But over the years the number of suppliers has reduced as companies have merged, taken over others or have closed. Hence Railtrack and Network Rail looking to other countries to see what other systems are available.
Train protection systems
The BR AWS (
Automatic Warning System) is integrated and standard for relay and computer based interlockings.
TPWS (
Train Protection & Warning System) has been retro fitted to existing interlocking systems and is now standard (where required) in new interlockings.
ATP (
Automatic Train Protection) now only used on parts of the Great Western main line, hence GWML ATP, is a computer based system overlaid on an existing interlocking system. It can (and was) used with the relay interlocking system and is currently used with SSI TFM (Trackside Functional Modules).
ERTMS is now in use in some parts of the country on some lines.
Compatibility with other interlocking types
Until recently, relays (BR930 series) were used to interface adjacent interlockings of different types, or of the same type, but controlled from a different signal box / PSB / Signalling Centre.
But more modern computer based interlockings are intended to have interconnecting data links.
Capacity
Very much depends on (a) what do you mean, train capacity or (b) system capacity?
The design of Route Relay Interlockings by their design use parallel logic so train (route/line) capacity is limited to the number of signals specified and the length of each signal (block) section at the design stage. The interlocking can handle all train movements within these limits.
The design and computing power of a SSI central interlocking is sufficient to handle up to 63 TFMs per interlocking. Bigger schemes just add additional central interlockings, each with their own (up to) 63 TFMs.
The more modern systems use much faster and more modern hardware. Hence have greater capacity.
Degree of standardisation of components
As time has progressed, attempts at increased standardisation has been attempted. Hence the BR930 series of relays has largely replaced most other types.
As I said earlier, SSI modules are interchangeable between the two manufacturers. And are used for existing SSI schemes and some of the more modern systems use SSI TFMs as well.
But as different systems have been used over the years, there are still plenty of modules or parts that are not interchangeable.
