Instead of adapting the bike to use the disused railway wouldn't it marvellous if the authorities converted them into cycleways just as the Spanish are doing throughout their country. Disused railway routes can get you straight out of the centre of big cities, avoiding heavy traffic and pollution. Their gradients are easy and they pass near centres of population ie villages and small towns. The largest concentration are in the north and south of Spain and by linking several together you can avoid some of the steeper terrain roads use. See what the Spanish are doing at with their 'Vias Verdes' (green ways):
In English
http://www.ffe.es/viasverdes/programme.htm
Typical route between río Guadajoz and Las Navas del Sepillar (Córdoba).
http://www.ffe.es/viasverdes/fichas_..._subbetica.htm
http://www.ffe.es/viasverdes/fichas_...subbetica1.htm
Route map at
http://www.ffe.es/viasverdes/mapa.htm