I think if electrification to Middlesbrough occurs, it will only be sensible / viable if the plan also includes Darlington to Saltburn & Stockton electrification. So, maybe by the 2030's ?
What? I Don't think so! What are you going to do with Darlo- Bishop? Why stop at Stockton? i mean no trains actually end there but lets make that the cut off point. That should allow for sensible diagramming of services. Hartlepooh might have been a better suggestion.
Boro will only get electrified as it is a terminus for TPE. Otherwise it becomes an appendix, an island of diesel operation in a sea of electric operations, and ripe for removal from their network. I have to say there is no real business case for it. Your case would have to be built on fringe benefits, operation of a single train type, reduced maintenance costs, streamlining depots operations etc and you need a LOT of those to make it worth the investment! You might also suggest the reputational damage to the railways in running 15-20 miles worth of diesel service under 100's of miles of wires.
It is possible that this, along with high level political lobbying, may make it happen. There is no other way that i can see as the business returns are simply not there.
There is no real freight based case, unless perhaps Teesport achieves its northern gateway ambitions and expands container traffic exponentially. Although the steel works at Redcar has turned out its 1mth tonne of steel in 5 months almost all of that goes out by ship form Teesport. The refinieries and chemical works of Teeside are not what they once were, more is the pity and ICI is long gone.
That said i think it SHOULD happen, simply to provide 100% coverage of the TPE network. It is the sensible thing to do but not the easy thing to do as you will spend a great deal for very little tangible reward.