Anything related to planning has to be demonstrated to be feasible before planning is granted.
Once planning is granted there will be legally binding planning conditions attached, if the planning authority are worth their salt there will be a time limit associated with any improvements (upon occupation of the 500th unit or within 10 years of the occupation of the 50th unit).
If there's no time limit then you will get developers building (if it's the 500th unit) 499 units and then walking away.
That’s not quite what I mean. In order for this to even be considered for planning, the local authority would first need to change its own rules about scale, height, density and type of development etc. It might be persuaded to do this on the basis of the benefits offered by the transport element of the scheme (which appear to be largely non-existent, as discussed above). So the local authority might be persuaded to change the rules to accommodate the scheme, and then find an application comes in within the new rules but without the railway. Probably badged as ‘we need to do the development first to pay for the railway’, but without a commitment to the latter.