In particular be careful if you are going to be reliant on Network Rail to deliver your obligations.  This article summarises a situation that Redrow found themselves in, having promised New Forest DC via a section 106 agreement to build a footbridge over an adjacent railway before the occupation of 125 open market dwellings on a site.  This effectively sterilised 17 dwellings on the site until the footbridge was constructed.  Redrow were obviously over-optimistic as to how long it would take them to get Network Rail's approval to the footbridge, as the remaining houses were ready to be sold and there was still no sign of the bridge.  Their attempts to minimise the obligation, by either reducing the number of units or replacing the obligation with the payment of a cash sum for the Council to do the works were rejected by the Council and an Inspector, and a High Court challenge to the Inspector's decision failed.

The moral here?  If you are offering section 106 agreement restrictions which are reliant upon the future co-operation of a third party bear in mind the risks involved if such consent is delayed, as there is no automatic easy way out of them.