North Downs Steam Railway? Never heard of it! Surely you mean the North Downs Line? No it wasn’t that! Where was this other railway then? It was a short-lived heritage line in North Kent, based not far from the southern end of the Dartford tunnel.
The NDSR began life with plans for the preservation of the disused Gravesend West branch. That line as built constituted a route north from Fawkham Junction (on the Chatham main line) to Gravesend West. It opened in 1886 and closed to passengers in 1953, but used for freight until 1976.

The NDSR as seen from Cotton Lane. The Metropolitan Railway’s T stock can be clearly seen.
Despite closure, the Gravesend West branch’s route and stations remained intact. (See Disused Stations for pics & history.) This was encouraging for the NDSRS.
Unfortunately negotiations with British Rail was somewhat protracted, not helped by the fact the route of the line had, after closure, been utilised to carry a 33,000 voltage electricity supply cable to a sub station at Fawkham Junction for the Chatham main lines.
The society had its railway stock stored at various locations in Kent including Chatham Dockyard. It wanted to gather all these at one location, and since availability of the Gravesend West branch was not forthcoming, the society set up a working railway museum in a field opposite the former Stone House Hospital, on the outskirts of Dartford.
The NDSR was mostly sidings with a short running line of perhaps 300 yards. The extensive sidings included many types of steam and diesel locomotives as well as an ex Metropolitan Railway EMU (originally MW stock and later designated T stock) and a 1938 tube train. The NDSR operated its entire life at Stone Lodge and for that reason it was also known as the Stone Lodge Railway.

General view of the NDSR’s station at Cotton Lane.
In terms of the ongoing Gravesend West line saga, whilst the society continued its negotiations, a company known as Resco Railways Ltd began its own attempt to take over the branch. This left the society feeling rather dominated.
Resco was a Woolwich based company whose expertise had begun with the restoration of the four wheeled District Railway carriage no.100. (This is normally kept on the Kent & East Sussex Railway but has been back to London for the 150th anniversaries of both the Met and the District Railways.)
Resco offered the NDSRS the option of running its trains over the newly acquired Gravesend West branch should the company’s purchase of that line be successful. The NDSRS refused the offer. After all Resco had muscled in on their plans and it just wasn’t acceptable.
Any hope of a heritage operation on the Gravesend branch was given up altogether. In the event this was somewhat of a blessing as the route of the former branch line was eventually requisitioned for a brand new railway!

Robert Stephenson & Hawthorn 0-6-0T and an industrial shunter. Note Dartford’s power station chimney at right.
A large number of these 0-60Ts were used on the private railways within Britain’s power stations. This page details the actual locomotive’s history and its current status.
The society was initially quite happy with its site at Stone Lodge. Alas it soon became subject to all sorts of attacks by vandals. The last straw came after an arson attack in 1995. This forced the NDSRS to end its operations. Its stock was moved to Tunbridge Wells West and the society merged with the then Tunbridge Wells and Eridge Railway.
My photographs, which I re-discovered recently, show the NDSR in October 1988, just a year after the heritage centre had opened.
It wasn’t a very accessible site, with visits being easiest made by car. Our visit was made on one of the many trips we made from Ilford down through the Dartford Tunnel into Kent. The new Dartford bridge had not yet been opened at the time.

Google Street View September 2008 showing the NDSR site. Is that an enthusiast who has come to see where the former railway once stood?
The area has now changed considerably thus it was somewhat difficult to identify the actual site. As the above image shows, the older Google Street views did at least show the area before any changes occurred. The wooden fencing seen in my 1988 photographs was still extant twenty years later. Most of that fence has now gone.
The site of the NDSR itself has been levelled out and is now the home of the Dartford Valley Community Rugby Cub, consisting of a rugby playing field, a large new stadium building and community centre.

The NDSR site is now the home of the local rugby club. Note the new houses on Cotton Lane. Source: Facebook
Some of my more astute readers will no doubt realise the aborted Gravesend West railway line is now part of High Speed One. The alignment between Fawkham Junction and Southfleet was taken over for the connecting spur from HS1 onto the Waterloo lines. This was regularly used by Eurostar trains when the first part of HS1 from Southfleet to the Channel Tunnel opened. The connection has been retained for maintenance trains to access HS1.
What of Resco Railways Ltd? It became an accredited steam and diesel heritage locomotive examiner for Railtrack. One such examination was for Deltic 9016 Gordon Highlander in March 2003. Industrial type diesel locomotives were also supplied by Resco for contract work.
The company too supplied spares for heritage locomotives and rolling stock including wheelsets and also built replica heritage stock. One example was the Science Museum’s Iron Duke locomotive. That operated in Kensington Gardens during 1995. In 2009 Resco collapsed and was wound up in 2013. See here and here.