The train would be cheaper. Price for 2 return tickets is about 50 GBPs each, I have no idea about rental costs but imagine it'd be more than the train even without petrol expense. Never sure why Americans worry so much about the cost of petrol as it has to be one of the least expensive costs in an itinerary unless one is driving enormous distances over a considerable period of time.

For more train information/costs/schedules www.nationalrail.co.uk

we will not be renting a car as we will be using a friend's car. The cost of petrol in Scotland is quite higher than in the US. That is why I'm asking.
I had thought train tickets from Roy Bridge to Edinburgh would be cheaper that 50 E return pp. At 100E, it would seem driving is cheaper.

To get from point A to point B (that is Spean Bridge BTW) for one person a train a train will be cheaper. For two people it partly depends on the route and how far in advance one books - could be cheaper or could be more expensive.

But for sightseeing and touring -- driving is often much easier. Trains do get one from point A to point B -- but seldom to all the castles/gardens/viewpoints in between. So it isn't a cut and dried calculation.

Oh - heck -- didn't see your second post. if you aren't paying to rent a car - then driving would probably be cheaper. But if you just want to get to Edinburgh w/o visiting sites along the way, a train is easier because you'd end up in the very centre of the city and not have to deal w/ parking.

Well, I didn't realize you weren't planning on renting a car. That certainly makes it cheaper to drive. I am aware that petrol is more expensive in the U.K. About twice as much. I just checked ticket prices and it's 24 GBPs ( not E) for a single ticket. It is actually cheaper to but two singles rather than a return. Total price for two people 96 GBPs. As janis said, driving will be cheaper but if only going to Edinburgh for the day, it might be worth the extra expense to avoid parking etc.

Thank you all for your input. I finally found an online site that calculated fuel cost for the trip. It was half the cost of the train at almost 25 GBP one way driving for two people compared to 48 GBP one way for two train tickets.
And by GoogleMapping it I saw that driving also took significantly less time than the train. Like 1 1/2 hrs less . So it would seem that even allowing for parking expense, driving is the better bet all around.
Although, there is a lot to be said for sitting on the train without the stress of navigating a long drive on the opposite side of the road. Even though I've driven in the UK many many times, it's still stressful.
But now I've got my answer.

Americans are always worried about the cost of gas since your costs about 3 times as much as ours - and our vehicies use much more than yours do - so it and seems like a potential huge expense to many people.

just a hint -- do NOT expect to do anything like the drive times from the on-line calculators. For example AA's route planner shows Spean bridge to central Edinburgh as 136 miles and 3 hours -- and googlemaps / 137 miles & 2 hr 55 min

Whereas it will take closer to 4 hours.