Few heritage railway societies can harbour such ambitious plans as those currently being implemented by the Swanage Railway Company. Not content with having relaid nearly 10km of tracks between Swanage and Norden in the beautiful Isle of Purbeck and running a full-blown regular steam passenger service along it, the society now hopes to extend the tracks a further 2.4km to link with the main rail network. And this is not just to allow it to run occasional steam specials into Bourne- mouth or beyond, as Swanage Railway business development co-ordinator Mark Stone explains.
'We hope to persuade a train operating company to run commuter services over our track to link Swanage with Poole and Bournemouth and beyond. This society is about running a railway - not just providing a museum for steam enthusiasts.'
Swanage already operates what must be the only serious steam-powered park and ride scheme in the country. The Isle of Purbeck is a popular tourist destination that suffers heavy road congestion in the summer. Drivers wanting to explore Swanage and Corfe Castle can already leave their cars in a Dorset County Council-run car park at Norden and use the trains instead. So successful has the scheme been that the local authority is currently enlarging the car park to double its original capacity. Extending the line and making it an attractive commuter alternative will be rather more complicated.
Steam services are limited to 32km/h, but to get commuters out of their cars the society believes the proposed diesel service must do 65km/h at least. The challenge is to convince a train operating company that services can operate safely at these higher speeds. But this is where the problems start.
'By the time British Rail closed the line and lifted the track in 1972, it hadn't done any serious maintenance on it for many years,' Stone says. 'Since we started relaying track more than 20 years ago, we've replaced three cattle underpasses and we have a full-time permanent way gang working. But there's a lot to be done before we can approach a TOC.'
The first real step towards this was taken in September last year with the formation of the Purbeck Rail Partnership. With a membership that includes local authorities, the Countryside Commission and Railtrack as well as the Swanage Railway, the new body's task is to upgrade the structures, trackbed and signalling on the line to the point where serious negotiations with a TOC can be launched. Its first move was to commission Aldershot- based Card Geotechnics to carry out a preliminary survey of the trackbed and structures. This has just been completed, and although the results are still being analysed, first indications are that the final report will be far from doom-laden.
Card Geotechnics director Dr Geoffrey Card says: 'Although most of the structures on the line date back to 1885, they were built to carry heavily- laden clay and stone wagons and are generally in good condition. The 30 or so bridges are generally Portland stone masonry arches, and only three underbridges have any significant displacement of the keystones or arch distortions.'
The main structure on the line, the four-span Corfe viaduct in the shadow of the ruined Castle, will only need waterproofing under the track bed. A more worrying challenge is posed by the only significant rock cutting, a short distance to the east of the viaduct, where the Purbeck stone is spalling and several large stone chunks in the 30m high cutting walls are threatening to fall down on to the tracks.
'Basically, the problem has its roots in the large cracks left by the original blasting operations,' Card explains. 'Freeze-thaw cycling did the rest. The loose blocks of stone will probably have to be removed soon, as the cracks are far too large to repair.'
Immediately beyond the 100m long cutting is a particularly tricky problem. During the years the line was closed, an old stable that butted up to an overbridge was converted into a dwelling house. Now its gable wall may be threatened by the increased dynamic loadings the proposed services would produce. So far, the engineering team has not come up with a solution.
'Our real problem, however, is drainage,' Card points out. 'The line is built on Kimmeridge clay, which was also used for the embankments, and Kimmeridge clay is very plastic.
'In some areas the old clay workings create ponding problems, and under some of the embankments the drainage culverts are blocked or collapsed and the embankments are slipping badly.'
In British Rail days, when the base of the embankment began to spread or slip and the trackbed subsided, BR simply lifted the track, spread more ballast and replaced the track on top. Swanage followed the same practice, but something more permanent is needed now. 'And it will have to be a low-tech, high labour content solution which can be built mostly by volunteers,' says Card.
'This will probably involve soil reinforcement and replacing defective culverts with clay pipes. Luckily, this approach will get round severe access problems in some sections.'
Beyond Norden lies the short gap to the nearest section of the rail network at Furzebrook. The freight-only line from BP's natural gas terminal at Furzebrook to Worgret Junction once formed part of the Swanage Branch of the London & South Western Railway and has been kept in working condition. But the unused structures and trackbed in the gap are known to be in a somewhat worse state than those further east. And just east of Nordern, an old overbridge that once carried a horse-drawn tramway - it brought clay from the borrow pits to the railway - is in a parlous state despite its listed status.
However, the enthusiasts who run the Swanage Railway are quietly confident their dreams can be realised in the near future. Something like 120,000 passengers travel the line every year - and it could be many, many more soon.