Welcome to the forum - and what an important post to start with, and a set of questions very well asked.
Some more background / inputs / thoughts. I have numbered these for easy back reference.
1. The ticket prices from Trowbridge to Swindon are also valid via Bath Spa. Until 7th December, with trains from Swindon at 06:12 and 18:44 (only), and from Trowbridge at 07:10 and 19:38 only, the majority of passengers using one of the direct services as part of their commute have 'doglegged' in the other direction. You'll meet different passengers on the direct morning and afternoon trains, as 11.5 hours in Swindon, or 13 hours in Trowbridge, is simply too long for a working day and that was the only direct option given. With the addition of further six trains each way per day (4 x the number of trains), the number of day trip return options direct explodes. Here's a diagram comparing Trowbridge to Swindon options - with the current one in red, and ones added from 9th December in black:
2. The journey time from Trowbridge to Swindon varies from 34 minutes on direct trains, to twice that (and more) by bus. There's a comparison
[here]3. Passenger numbers from Trowbridge to Swindon aren't enough to justify a long train, nor a none-stop one, so the cost of provision per seat-mile, including the staffing cost and how long the train takes to cover a mile as well, is going to be high.
4. There's no way that a train running without permanent taxpayer input is going to compete with the bus with 100% taxpayer funding for travelers who travel on concessionary passes. I appreciate that the original post wasn't talking about this group - senior citizens - but it's a point to bear in mind - "reductio ad abusrdum" if you like. I wouldn't expect the train to make any significant inroads into the number of pensioners using the 49 bus service due to the cost, and due to the completely different routes served to that it's only through Trowbridge - Swindon traffic that is being provided with the new train option.
4a. If you're accounting for your cost of travel, how do you value your time? Is your time worth - say - 15p per minute to you? If so, your real cost (for a day return, peak) will be 17.60 + .15 x 70 for the new train service, or 5.70 + 0.15 x 160 for the bus. That's 28.10 (train) v 29.70 (bus)
5. I don't think that the 11.54 for the car journey includes the cost of parking in Swindon. With cost of time as in the previous paragraph, you're looking at 27.44 + Parking cost. (I am meeting with Connecting Wiltshire folks on Friday - I can check whether their pricing include parking if I get a chance)
6. Bus and train fares when looked at in terms of pence per mile are a bit of a mess ... and you're seeing that on this particular journey comparison. That does sometimes leave you with extra options you may not have looked at. I believe that for most Trowbridge to Swindon rail passengers, the benefit of being able to travel via Bath Spa on occasions that there's no direct train will be useful, and so the dual route use will continue to be useful. But you could travel at a lower cost by train if you were to purchase Trowbridge - Melksham and Melksham - Swindon return or season tickets. That's not going to compete with the bus fares, but if your place sufficiently low a value on your time for a wait to be better than paying more for more options, it's worth considering.
7. I use the train a lot, and will be using it more. I could drive, I could take the bus. Driving, I end up tired and irritable at the end of the journey, and I've not been able to work / read / sleep on the way. I could use the bus, but I'm not really able to work or read there. Train time isn't wasted time to me; I complete the journey having not seen my 15p per minute spent at all - it's not time taken from other things - and the train ends up cheaper for me in real terms.
Bear in mind on the leisure fares that 4 can usually travel for the price of two. Did your car figures there include London and Oxford parking costs? I look forward to meeting you on the TransWilts trains; they're not going to be for everyone making Trowbridge to Swindon journeys, but they are going to be very attractive to many people. The Community Rail Partnership is keeping an eye on ticket costs, special deals, etc, and we have had conversations with
FGW▸ already on some matters. Not necessarily in the direction of "please reduce prices" but rather more along the lines of "remove nasty expensive quirks, but make sure it's economically sensible to run the service too".
Edit to correct typos