Apparently it's not actually closed off (yet).
I got an official response from tie today -
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In response to the question raised: "why on earth can't they just put in some steps from the path to the road, beside the construction site"
As you surmised in your initial response the limiting factor relating to the provision of such a set of steps is that they would be too steep. The Council are not able to approve something which is so far removed from their standards and therefore clearly not compliant with disabled access requirements.
Additionally it should be noted that the structure being built here is a complex one being built in a difficult site as it is immediately adjacent to the live railway, hence the contractor will require more working space than may initially be obvious and restricting the scope for access off of Russell Road as I expect the writer of the initial query envisaged.
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I am aware of the difficulties of working near a "live railway". Clearly public safety is paramount, though I dont think Network Rail, RailTrack, and predecessors are always entirely co-operative...
As for ramps/steps etc, not being an engineer I can't comment on "wills" or "ways"...