I've been searching fares for various journeys on the Nat Rail website and no matter what destinations, times, or days I search, the end result is that two one-way tickets are always cheaper than a round-trip fare, which doesn't quite seem right.

nope, quite normal.

okay, cool, thanks!

I'm sort of surprised; there are indeed plenty of cases where two separate single tickets are cheaper, but I'd be very surprised if it were true ALL of the time.. what routes/dates/times have you been looking at?

I vaguely remember that either National Rail or the Trainline have a banner on their site saying "Two singles fares MAY be cheaper", but when I've looked at fares in more detail, it hasn't proven to be the case... so just curious...

It is frequently the case that singles can be cheaper than returns. Websites such as www.TheTrainLine.com even have it suggested during searches on that website.
Another anomaly is that it is sometimes even cheaper still to buy four single tickets as if splitting the journey but never leave the train. Crazy or what!

Have a look at http://www.moneysavingexpert.com/tra...-train-tickets

meks, that's a good URL you provided. I have twice been able to save money by buying separate tickets for parts of the journey, but in both cases because there was a need to change trains and train operators. Once was travelling from Abergavenny to Liverpool. Separate tickets from Abergavenny to Crewe and Crewe to Liverpool were about £10 cheaper than an Abergavenny to Liverpool ticket. I saved a bit more than that by buying tickets Swansea to Fareham and Fareham to Chichester instead of a Swansea to Chichester ticket.

Ron,
Pleased that it was of interest. I know that common sense is out of the question when it comes to rail companies and just hope that the explanations etc on that website can be of use to others planning future journeys.
I know it seems strange but it will undoubtedly save other users money.

I searched London to Morecambe, London to Lancaster, London to Stansted, London to Oxford and the reverses of those journeys. With Stansted, the round-trip fare was exactly double the single fares, but with all the others, two single fares were cheaper. I should add that I always search off-peak times. Thanks for all the great information!