Have a look at the WCML. Plenty of freight trains running purely on electrified lines but diesel hauled throughout, such as Mossend - Daventry. Of course not many freight terminals are actually electrified because that restricts loading methods and it's cheaper to diesel haul throughout than have to use a separate diesel shunter at each end. I suspect we are very many years away from seeing any worthwhile increase in electrically hauled freight.
Electrically hauled freight will increase slowly, maybe a little more rapidly if the Class 88 proves to be a successful design, as it removes the need for electrified reception roads and diesel shunters.
Switzerland manages very well with battery shunters which charge from the overhead lines, IPEMU style. The next test for this will be what happens at Grangemouth and how much, if any, of the port railway is electrified by Forth Ports.
The good news for electrification of freight is that power that's available - the Class 88 is a 4MW locomotive, the Class 68 just 2.8MW, that translates into faster acceleration, higher top end speeds where stock can manage that, and more flexible pathing options where an electric loco can get from one loop to another more quickly, so less of the peak periods are off limit to freight.
The pathing flexibility, ability to haul longer loads, the higher speeds and the reduction in emissions is ultimately likely to result in Government initiatives to replace diesel locomotives with electric ones.
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It's also worth remembering that whilst some electrification projects won't see much if any services convert from diesel to electric, they'll have to be done if you want to keep pushing further north, south, east and west to the extremities of the country. It might be the case that on a few lines, we have just one or two bi-mode units making use of the OLE every hour for the first 5 or 10 years, until wiring spreads further.