So does this mean that they'll only be two specific services that will be worked by a HST? I'm not very enlightened on the subject of "diagrams", so I'm easily confused when diagrams are mentioned.
Unfortunately, 'diagrams' is a word with several possible meanings in railway discussions:
- a plan-view drawing of something (this is what I would consider to be the accepted definition in general use, but the rail industry isn't limited to that definition)
- a list of train workings/services which are planned to be operated by the same loco/unit/rake-of-LHCS (rolling stock diagram) or member of staff (train crew diagram) - this is probably the meaning that applies in this context
- a specific design of rail vehicle (for example GH1G, which I think is one of the many diagrams (for want of a better word) of mark 3 kitchen/buffet car) - a mark 4 TSOE I think would be a different diagram from a normal mark 4 TSO - this is a meaning I have only recently come across so I'm not sure I have understood/explained it correctly
To further elaborate on the second definition above (which it appears is the one you're interested in), imagine a route with a train every 3 hours, where trains take about 85 minutes to complete the run. It might (but probably wouldn't, because the way I have done it here is resource inefficient) have a timetable something like this:
| ONE | TWO | ONE | TWO | ONE | TWO | ONE |
Station A | | 07:00 | 10:00 | 13:00 | 16:00 | 19:00 | 22:00 |
---|
Station Z | | 08:25 | 11:25 | 14:25 | 17:25 | 20:25 | 23:25 |
---|
Station Z | 05:50 | 08:50 | 11:50 | 14:50 | 17:50 | 20:50 | |
---|
Station A | 07:15 | 10:15 | 13:15 | 16:15 | 19:15 | 22:15 | |
---|
In this case that service would require two diagrams to operate (with buses, this appears to be called the Peak Vehicle Requirement (PVR) of a route). I have intentifed the two unit diagrams as
ONE and
TWO in the above table, but I suspect it is more-common to list the workings. For example, diagram
TWO would probably look something like this (I don't work in the industry, so don't know what it really looks like):
- 07:00 Station A to Station Z
- 08:50 Station Z to Station A
- 13:00 Station A to Station Z
- ... and so on until...
- 20:50 Station Z to Station A
Since my timetable is so resource-inefficient, with units sat around at Station A for hours between runs, there's a good chance that the diagrams would be interworked with another route. For example, diagram
ONE could start like this (station C could be anywhere, not necessariliy between A and Z, but for this example assume it is about 20 minutes from A):
- 05:50 Station Z to Station A
- 07:30 Station A to Station C
- 08:00 Station C to Station A
- 08:30 Station A to Station C
- 09:00 Station C to Station A
- 10:00 Station A to Station Z
- ... and so on...
Hope that helps.
In the case of the XC Plymouth-Edinburgh route, it is such a long journey that a single unit (or HST set) would be unable to get four trips (two services in each direction) done in a day. Thus one diagram for the XC IC125s (which don't generally stray from that route unless there is engineering work somewhere) can include a maximum of three specific services (two in one direction and one in the other) if it starts very early in the day but normally seem to only include one or two specific services.
'Two diagrams' means 'two sets are
planned to be in use', so depending on how many services are included on the actual diagrams, it could be that four services (two southbound and two northbound) will be worked by IC125s (eg. in recent years the norm seems to have been one set doing an Edinburgh-Plymouth run and then coming back on a Plymouth-Leeds and the second set doing Leeds-Plymouth southbound then heading back north to Edinburgh). However, I understand that maintainance for XC's IC125 is done in Plymouth so it may be that they reduce it two one service per diagram (ie. the first set/diagram being an Edinburgh to Plymouth working, with the set then going on depot; and the second set/diagram coming off depot in Plymouth and going north to Edinburgh to replace the other set ready for the next day).