If like me you have a fondness for both railways and archives then you might be interested in Network Rail’s Virtual Archive, which is a collection of documents (mostly plans and drawings) from the organisation’s archive.
The Network Rail Archive is not open to the public because they are “fully engaged in managing information to support the activities of the operational railway“. This is perhaps the most important thing to remember here, these may be historic documents but many of them are also engineering drawings for bridges and stations that are still in use today.
The documents are presented as a series of articles, along with historical background on the subject, such as a timeline or links to information on the railway company or engineer responsible.
This is a wonderful collection of documents, but there is probably little chance that the average family historian will find any information about their ancestors in the archive.
However most of these bridges, stations and tunnels were built by armies of labourers, if your ancestor was one of these labourers then the documents in the archive might give you a chance to see what it was they were working on.
The only drawback so far is the lack of documents from my neck of the woods. I would love to see some plans for Balcombe Viaduct on the Lonodn to Brighton mainline or details of some Sussex railway stations like Brighton or Bognor Regis.