It's interesting to look in the location directory in TRUST to see how all this is set up on the internal systems. For each station you have the full location name (which is not necessarily the official station name), the Tiploc (up to 7 characters, used in planning systems), the familiar 3-letter CRS station code, the Stanox number (used by TOPS), and the Stanox name (maximum 9-character version of the location name which is the abbreviated location name displayed when looking at TRUST - shown as "Stanme" below).
Location name BLETCHLEY
Tiploc BLTCHLY
CRS code BLY
Stanox 70306
Stanme BLETCHLEY
Location name BLETCHLEY HIGH LEVEL
Tiploc BLTCHHL
CRS code BLU
Stanox 70317
Stanme BLTCHLYHL
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I remember being puzzled by the location search function in TRUST (which is similar to the search function on Realtime trains) when looking at Reading. If you type "READING" as the location (instead of RDG for example), you get an error message saying "groups not currently supported." It turns out that Reading station is actually split into two within TRUST, with the 3rd rail bay platforms being a separate location RDZ.
I suppose setting it up like that gave an easy way of getting information from TRUST that focused either on the Southern Region or Western Region services, which were operationally distinct, without having to filter the results by train operator. With the platform numbers of the third-rail bays being numbered in sequence with the rest of the station and physically adjacent to it on the same level, I doubt it would ever have seriously been considered as a separate station (at least during the timeframe when TRUST was around - apparently Reading Southern was a separate station that closed in 1965, with services being transferred to Reading General).
Location name READING
Tiploc RDNGSTN
CRS code RDG
Stanox 74237
Stanme READNG
Location name READING
Tiploc RDNG4AB
CRS code RDZ
Stanox 74236
Stanme RDNG 456
For a location search in TRUST, you have to pick RDG or RDZ. When doing the same thing in Realtime trains, it combines them into one station under the RDG code to show all services at the station. Realtime trains also shows the coach link to Heathrow - there is an RBU code for buses at Reading and the schedules are set up as 0B00 (train category BB in TRUST, "Bus").
Location name READING BUS
Tiploc RDNGBUS
CRS code RBU
Stanox None
Stanme None
When doing a journey enquiry in TRUST (similar to the Journey Planner on National Rail Enquiries), using the name "READING" will show journeys to either part of the station, as appropriate. It groups them under a "pseudo-Tiploc" of RDNGGRP (Reading group) which is the group that wasn't supported on the location search.
Location name READING
Pseudo-tiploc RDNGGRP
Stations in group: RDNGSTN, RDNG4AB
In TRUST, you can still specify a journey to one particular part of Reading station (RDG for the main line and RDZ for the 3rd rail bay platforms), but for example if you try to find a journey from Ealing Broadway to RDZ, it will just give you a journey to RDG and tell you to walk to RDZ (with a zero minute walk time), so you would be better off using a "via" option to specify a particular route.
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At Tamworth, it makes sense to treat the two halves of the station as separate locations, at least for operational reasons within TRUST. For example, when that
Turbostar unit was stuck in the platform for a few hours back in April, having a separate TAH location for the high level makes it easier to quickly get a list of trains affected due through the high level platforms, while ignoring unaffected services through the low level.
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Lichfield is an interesting case as it includes three locations in a group - the high and low level at Trent Valley plus Lichfield City:
Location name LICHFIELD
Pseudo-tiploc LICHFLD
Stations in group: LICHFIELD CITY, LICHFIELD TRENT VALLEY HL, LICHFIELD TRENT VALLEY LL
A journey enquiry to "Lichfield" will show journeys to any of the three, whichever is the more logical route to arrive on (and shows times at both Lichfield City and Trent Valley for trains that call at both).
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Smethwick is similar, with separate locations for the high and low level at Galton Bridge, plus a "Smethwick" group which includes Rolfe Street.
In Realtime trains, the high and low level at Galton Bridge are combined under SGB.
Location name SMETHWICK GALTON BDG L.L.
Tiploc GALTILL
CRS code XGQ
Stanox 65822
Stanme GALTONBLL
Location name SMETHWICK GALTON BDG H.L.
Tiploc GALTINT
CRS code SGB
Stanox 65900
Stanme GALTONBHL
Location name SMETHWICK ROLFE STREET
Tiploc SMTHKRS
CRS code SMR
Stanox 65824
Stanme SMTHWKRST
Location name SMETHWICK
Pseudo-tiploc SMETHIK
Stations in group
SMETHWICK GALTON BDG L.L. SMETHWICK GALTON BDG H.L. SMETHWICK ROLFE STREET
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Richmond has a separate location for the North London Line part of the station (XRI). In this case there is no separate "group" set up. The same is also true for Highbury & Islington, which has 3 CRS codes but no group combining them. Despite this, using the journey enquiry in TRUST, a journey from HHY to RMD (through platforms at Richmond) still gives the sensible option via the North London line - it just adds zero minute walks from HHY (Moorgate platforms of Highbury & Islington) to HII (North London line platforms) at the beginning and from XRI to RMD at the end.
Location name RICHMOND
Tiploc RICHMND
CRS code RMD
Stanox 87135
Stanme RICHMOND
Location name RICHMOND NLL
Tiploc RICHNLL
CRS code XRI
Stanox 87132
Stanme RICHMNDNL
Location name HIGHBURY & ISLINGTON
Tiploc HGHI
CRS code HHY
Stanox 54293
Stanme HIGH&ISGN
Location name HIGHBURY + ISLINGTON
Tiploc HIGHBYA
CRS code HII
Stanox 52053
Stanme HIGH&ISNL
Location name HIGHBURY AND ISLINGTON ELL
Tiploc HIGHBYE
CRS code XHZ
Stanox 52051
Stanme HIGHBYELL
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So even if the Bletchley main line and high level platforms are two separate locations in TRUST, by the time East West Rail opens this should be combined into a single location on public-facing systems under BLY (as per post #7 and #8) as different platform numbers of a single station, with the distinction between high and low level hidden from view unless you are looking directly at TRUST or some other internal system.
On the topic of walking times, TRUST doesn't seem to add any extra walking time between different parts of stations that are officially considered one station with the same name (although the official interchange time at these stations should include an allowance for this). For example, Glasgow Queen Street high level to low level platforms is shown as a zero minute walk, but Glasgow Central (high or low Level) to Glasgow Queen Street (high or low level) is shown as a 20-minute walk, and London Waterloo to Waterloo East is shown as a 16 minute walk. As mentioned in post #16 I think Waterloo East (WAE) is officially considered as a separate station to London Waterloo (WAT), for example it has a separate entry in the station information section of the National Rail website, they have distinct names on the station signage, and one is managed by Network Rail and the other by Southeastern. The lettering of the platforms at Waterloo East is an interesting oddity - on the one hand it's obviously desirable to have platform numbers that cannot be confused with the main Waterloo station, but officially being separate stations would have made it odd to give Waterloo East platform numbers that followed on from the main station, e.g. platform 25. In contrast, somewhere like Glasgow Central has the low level platforms numbered in sequence with the rest of the station because although they are physically separate, they are officially part of the same station.
Stations in the list in post #18 have 2 or 3 CRS codes, but (unlike the situation at Waterloo) only one is recognised by National Rail Enquiries, and are officially a single station with a single name (e.g. Tamworth is the official station name rather than Tamworth High Level and Low Level).
As for a problem with the CRS code at Bletchley mentioned in post #2, attempting a journey enquiry in TRUST from BLY to BLU (which should tell you to walk, i.e. from the main line platforms to the high level platforms) results in a "No reasonable journey found" from Bletchley to... BLUNDELLSANDS & CROSBY.
I assume the code for Blundellsands & Crosby was changed from BLU to BLN to avoid a clash with Bletchley high level platforms, but that hasn't quite filtered through to the journey planner within TRUST. So be careful at Bletchley when East West Rail opens, there could be some sort of portal that opens up and teleports unwary travellers on an unreasonable journey to Merseyside!