It would be nice for rail to be cheaper. But in practise it's never going to happen unless driving costs £2 per mile or something ridiculous.
Consider this: last month, I went on a mini-holiday photographing the GBRF railtour in (mostly) Scotland, by car. Two thirds of the total cost was petrol, £120 in total at 60mpg. I did about 1200 miles, from Worcestershire to Glasgow (via several stop-off points in Cumbria to investigate potential photographic locations), Glasgow to Oban then Carlisle (with a total of 10 photo stops), and Carlisle to London to Worcestershire (via a couple more stops). A comparable journey by rail (ignoring the fact that it would be impossible to check out lineside photo stops, overtake the train as it winds its way through the Highlands at an average speed of under 30mph etc if travelling by train) would cost more and significantly reduce the scope of my trip. Using flexible tickets, TrainSplit will give me a cost of £105 for Birmingham-Oban return, to which I'd have to add the costs of getting to Birmingham, plus the dog-leg to London. This is actually a bit cheaper than I was expecting, but if I didn't have a Railcard, it would be far more. Plus, as I've already mentioned, for this trip the car gave far, far more flexibility.
Now, let's say that I went with a friend. My fuel cost would halve, but my train tickets would cost the same. No contest!
I enjoy train travel, hence I go on various leisure trips when time allows. I also enjoy driving, but I don't go on driving leisure trips (ie: going out for a drive on a Sunday evening because I can) in the way that I do by rail. Perhaps this indicates that rail travel doesn't need to be any cheaper?
(Incidentally, I recently discovered that a seal in the engine on my car was broken, resulting in exhaust gasses leaking out and being sucked into the passenger compartment by the fan. Probably been like that for several months or more. Not something you want if you're doing 1200 miles in 3 days! At least on a train you generally don't have to worry about stuff like this...
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