For the purposes of this audit railway linesides are the vegetated lands that lie adjacent to operational above-surface railways. Closed railway routes – those that are no longer in railway ownership – are not included. Vegetated lineside land may include embankments, cuttings, areas around stations and by junctions, above tunnel-mouths, and derelict sidings and marshalling yards. The habitats present are predominantly grassland, scrub, woodland and ruderal vegetation – wetlands are noticeable by their virtual absence – the key link is that they are all on land owned and/or managed as part of the railway network.

The railway network in London was largely created between 1836 and 1936, both stimulating and reacting to the rapid urban growth of the capital. Although the network cut rudely into open countryside when it was first built, most has subsequently become part of the urban landscape and, through the process of natural colonisation, now provides significant areas of wildlife habitat.

There are approximately 795km (492 miles) of open operating railway corridors in London, not including closed railway lines such as Horniman Railway Trail in Lewisham and Parkland Walk in Haringey, which are managed for nature conservation and/or amenity. The open corridors are owned predominantly by two companies; Railtrack Plc and London Underground Limited (LUL) and a number of corridors are used by both underground and surface rail trains. Smaller lengths of railway are owned and/or managed by Docklands Light Railway (DLR), Tramlink in Croydon and a few private industries. Most London boroughs contain between 10 and 35km of railway corridor; see Table 1 and Map b. Only four contain more than 40km (Bromley, Croydon, Lewisham and Brent), whilst two contain less than 5km (Westminster and City of London). Some boroughs have larger lineside networks than others proportional to their area. The best include Lewisham, Tower Hamlets (although much of this is raised DLR), and Newham; the poorest include Westminster, Redbridge and Camden. The total area of railway corridors has yet to be calculated, as has the total area of linesides of wildlife value.

In inner London the railways are mostly elevated on viaducts or in deep cuttings and hence support very limited biodiversity. Further than 5km from the city centre, the linesides become broader (usually as they meet ground level) and begin to support vegetation. Towards the London borders quite significant areas of semi-natural habitat can be included within the railway corridor. Lineside habitats are largely a legacy of the countryside they were originally built through, their subsequent management, together with the indirect impacts of railway operation. Linesides were once managed intensively and although in certain areas trees were planted to screen residential properties, the ‘railway’ poplar, Populus x canadensis ‘Regenerata’ for example, the majority were maintained as grassland. From the 1920s, with the change to electrification and the ever-increasing labour costs, management became more relaxed, especially so after the Modernisation programme of 1955. From the 1970s scrub and woodland began to appear on the more rural stretches, to the extent where many of today’s railsides support recent sycamore woodlands – often the only significant stands of woodland in many inner London areas.

Changes to the railway network and land area have been significant since the mid-1980s, and with privatisation development pressure may result in further land-take, particularly on derelict marshalling yards (although the growing trend for increased rail freight traffic may prevent this on certain routes). New railway projects have led to corridors being created, often at the expense of semi-natural habitat (e.g. Addington Hills in Croydon), but such projects now require environmental assessments and with heightened public sensitivity are unlikely to proceed without considerable ecological compensation.

The railway network supports significant areas of biodiversity importance in London. A total of 838 ha of lineside have been identified as Sites of Importance for Nature Conservation to date by the London Ecology Unit (LEU) (see Map a). The range of habitats (from chalk cliffs to early successional wastelands), together with their relative lack of human disturbance, provides a diversity of fauna and flora that in some areas can be relatively rich. In inner London they often support the only significant woodlands and rough grasslands. Sunny grass embankments may be havens for butterflies, grasshoppers, slow-worm and kestrel, whilst woodlands can support great tit, great spotted woodpecker and sparrowhawk. Derelict marshalling yards with a free-draining, alkaline substrate often support a diverse range of ruderal plants, before succeeding towards birch scrub and woodland. Temple Mills and Feltham are two fine examples, with a new species of spider to the UK, Zodarion rubidum, being recorded at the former site in 1999.

A number of plants and animals are characteristic of London’s linesides. Plants such as everlasting sweet-pea, rosebay willowherb and Oxford ragwort have spread through the development and operation of railways, whereas sycamore is the predominant tree species. Buddleia occupies lineside ballast and cracks in railway structures. Some ‘pest’ species such as Japanese knotweed and giant hogweed have also taken root, often in large monocultures.

Well-vegetated linesides will act as ‘green corridors’ and the combined network of railways will help to permit movement of some species along them between adjoining sites – either through direct movement (e.g. mammals) or dispersal assisted by the movements of trains (e.g. seeds of plants). Thus railway linesides will add to and benefit from the ecological integrity of adjacent SINCs and other open green space. The value of green corridors has been recognised in PPG9, in that they "help form a network to ensure the maintenance of the current range and diversity of our flora [and] fauna" (para. 15).

A few lineside areas such as Gunnersbury Triangle in Chiswick, Gillespie Park in Islington and New Cross Gate Cutting in Lewisham are actively managed as nature reserves. Work by Railtrack and London Wildlife Trust to identify further nature reserves as well as priority ‘conservation zones’ began in 1997, but requires further development. LB Lewisham is also seeking to create a large railside Local Nature Reserve. A leaflet, ‘Wild Linesides’, was published in 1998 to promote the ecological interest of London’s railways to the travelling public.