Wife and I did Swanage railway Saturday. It was the Swanage marathon day, hundreds or maybe (low) thousands carted home to Swanage from Corfe Castle. DMU doing some mileage, but lots on the steam service early afternoon. Must be a good moneyspinner though quite a few unticketed travellers, asked to "make a donation" online. Nice railway though rather short
Depends on how you view a journey of 6 miles which takes about 20 minutes one way. The branch line joins the main line at Wareham which gives a length of 10 miles.
Swanage railway has operated two trials to Wareham, but the additional costs caused in part by a small amount of running on the main line are huge, compared to a small increase in revenue.
A longer journey time also means a less frequent service, unless more traction is operated which again increases costs. A longer track also increases the amount of permanent way maintenance, and may involve structures which are very expensive to maintain for example bridges.
Longer journeys increases the pressure to provide on train toilets which bring a whole new set of problems.
Journeys do have to go from somewhere to somewhere, terminating at a halt with nothing to really see or do may have been fine in the early days of heritage railways, but is far less appealing to discerning passengers today.
In my opinion, Swanage Railway is an excellent example of a heritage railway, that links two major tourist attractions in an appealing way.