Looking at the specification for the Hoddlesden Branch (ibid) again, the section on level crossings says in part: "All occupation roads to have oak gates for carts, and a curbed swing gate for foot passengers at each end, as shown on drawing. The field gates to be of oak, the same as occupation roads, but without the side gates. All the gates to have strong posts, hinges, latches, and padlocks."
Standard practice in the absence of enough information is to start with the simplest interpretation of what we do have. So maybe a field gate is a gate into a field, usually from a road but possible from another field.
Now, building a railway involved re-routing roads, and that would need new fences to be built. And field boundaries cut by the line, if taken out beyond the railway's boundary fences, would need some rebuilding. So is this kind of new fencing not along the railway's sides called up in your documents? And if so, are any gates in those fences required?
One further possibility is where (as must have been usual) several of a farmers fields were split by the line but only one occupation crossing was provided. New gates between fields might be considered necessary to restore "connectivity" between the now separated parts of those fields.
The padlocks could be a requirement (on all gates including at crossings) to cover the case where the farmer decided he did not need a gate after all. In that case he might demand only a fence, i.e. a locked gate.