I think I'd agree.
Looking at the rail, it seems to have been fixed in place separately to the existing floor judging by the two to three inches of cement either side. You may be able to chisel down at one end of the rail to see how deep that 'new' concrete is, and the rail may be fixed in with large screws onto the original flooring channel.
I'd try this first, I think. All you'd lose is a days effort, but multiply that effort along the whole length for the big picture..!
John's right, though. It's easier to screed a new surface without disturbing anything & raise the floor by a few centimetres to give a smooth new base. You have ample clearance below the side door step by the looks of it & the DPC is two courses high I think. You just ramp the entrance slightly to ease driving in and out.
A pre-mix concrete pour with fibres mixed in is ample strength at depth of about 25mm for that.
More concrete info
HERE 
