I'm watching the 07:00 from Westbury this morning.  It left there 11 late ... was 26 minutes late by Chippenham and shows a Swindon arrival of 08:11.  The onwards to Gloucester and Cheltenham, estimated arrival there just 9 minutes after it is scheduled, (it's allowed an incredible 19 minutes from Gloucester to Cheltenham).

1. Glad it ran

2. If it can gain 15 minutes from Chippenham to Cheltentenham, why can't it be retimed to start out quarter of an hour later in the first place? An 07:30 from Melksham to Swindon would be a lot more useful than an 07:17 - a little of the business might come back.

Probably because South Wales Trains were being diverted.  I also noticed / heard a lot of freight last night and this morning as well

The single track between Swindon and Kemble appears to be a stumbling block. Times at the moment are Melksham 0717, Swindon 0750-0752, Kemble 0807, Gloucester 0842-0850, Cheltenham 0901. But the 0715 Cheltenham-Paddington is due at Kemble 0807-0808 and Swindon 0822-0823. i.e. as soon as the Melksham train has arrived at Kemble the Paddington train goes on to the single line to Swindon. Running the Melksham train 15mins later would mean it arriving at Swindon at 0805 only 3 mins before the Paddington train goes onto the single line at Kemble. So in reality the Melksham train wouldn't be able to leave Swindon until the Paddington train has arrived at Swindon at 0822. The Swindon-Cheltenham leg of the journey would therefore be approx half an hour later than at present giving arrival times of approx 0910 at Gloucester and 0930 at Cheltenham which is too late for many commuters.

The fundamental problem is that the Westbury-Swindon and Swindon-Cheltenham legs of the journey are made by the same unit. If the two legs were run with seperate units it would probably be possible to run the Westbury-Swindon leg at a more convenient time.


Yes and no. There's a few minutes 'pathing allowance' in the schedule of the 0700 Westbury-Cheltenham but this is there specifically to make the timetable work. e.g. if you're planning a timetable and you find that you've got two trains converging on a junction at the same time then obviously one of them is going to have to wait it's turn. So you put some 'waiting time' (correctly known as pathing allowance) into the schedule of one of the trains. Now if the train with the pathing allowance was 10 mins late then by the time it got to the junction the other train would be long gone - so the pathing allowance wouldn't actually be needed. The late train therefore makes up some time.
So for the Melksham train this morning because it was late it probably didn't need some or all of it's pathing allowance and therefore made up some time (it also managed a very quick 2 min reversal at Gloucester (allowed 8 mins) which saved 6 mins of the delay). But when the Melksham train is running on time it *will* need it's pathing allowance to avoid conflicting with other trains.

Timetabling trains is harder than it might appear. For a Westbury to Swindon journey I see the following constraints that could affect the timings:
1) Between Westbury and Bradforth South Junction (just north of Trowbridge) you have to run at a time that fits in between the existing Westbury-Bath trains.
2) At Bradforth South Junction you then have to cross the southbound track to gain access to the single line to Melksham. This can only be done during a 'gap' between southbound trains.
3) You can only go onto the single line if there's nothing coming the other way!
4) At Thingley Junction the single line joins the Bath-Swindon line. To get onto the northbound track you need to cross over the southbound track so again this can only be done during a 'gap' between southbound trains. You also have to ensure that you're joining the northbound track during a gap between northbound trains.
5) You then join the main Bristol Parkway-Swindon line at Wooton Bassett Junction to the west of Swindon so you'll need to make sure you're in a gap beween the Bristol Parkway-Swindon trains. You'll also be crossing over the westbound track at this junction so you need to be in a gap between westbound trains as well.
6) At last you've finally made it to Swindon. Let's just hope there's an empty platform available for you at the time you've arrived..... :-)

I hope that gives everyone an insight into how difficult timetabling is. I don't compile timetables for a living so there's probably a lot more constraints that I'm not even aware of. You can see that even altering the times of a train by a few minutes can have knock on effects over a wide area.


 

Steve, many thanks for your detailed inputs.   Yes, I agree with you that it's "not that easy" and indeed I should have put a smiley face on my original post as I knew it at the time too.  But your post and others are excellent background / fill-in information for others who are newish to this topic and who, perhaps, my original post mislead.

The core problem with timing the 07:17 later is that it's the train that's going on up and forming the Gloucster commuter service.   It's was the operator's choice to use the same 'set' for that - you'll see a lot of suggestions and other prior reports, including the DfT's Jacobs report, which didn't couple them in this way and came to much more sensible conclusions for "our" service than th eone that's been provided.   In a nutshell, First took a decision to squeeze money out of the local trains from Swindon by reducing train sets - just as elsewhere - and  can't provide (even with high availability) transport to meet all the key flows properly.  And they've decided that Frome / Westbury / Trowbridge / Melksham to Swindon commuters are one of the flows they won't meet. They even had the SLC2 relaxed at the last minute, long after the franchise was signed, to let them break their committment for the next 10 years.