Well, smokey has made some very useful suggestions elsewhere:
Cable theft is getting to be a big problem for Network Rail, the value of Scrap is the problem, however three things Network Rail could do,
1 On signal cable use "TINNED" (electro-plated) copper cores, drives down the scrap value no end.
2 Have Property of Network Rail Printed on Outside and Inner cores of cable, most scrapmen won't touch such cable without the correct paperwork.
3 On Power Cables (16mm core and above) use Aluminium cable instead of copper.
And, to be fair, theft of copper is not just a problem on the railways:
"A man was killed trying to steal copper cable which was carrying 11,000 volts, an inquest has been told. Kirk Thompson, 43, from Bettws, Newport, was electrocuted in April at the derelict Panteg steelworks, in Pontypool, Torfaen. The former heroin addict's bolt croppers pierced the plastic coating of a cable connected to the National Grid.
Recording an accidental death verdict at Newport, deputy Gwent coroner Wendy James said his "luck ran out". The deputy coroner added: "Never has the old saying 'crime never pays' been so graphically illustrated as in this case. These type of thefts are on the increase and people should heed the warnings of the dangers of electricity. If they choose to ignore the warnings they will pay the highest price possible."
For full details, see
http://news.bbc.co.uk/1/hi/wales/7623021.stm