The required mechanical and electrical equipment, for diesel propulsion alone, took up a sizeable space; adding a bimode capability would have further eroded revenue earning (passenger) space.
Also, pre 1973-ish, fuel was cheap and environmental factors hardly considered (diesel was “clean” when compared to steam) so running on diesel over the third rail (on Hastings and Oxted lines runs in particular) was not an issue.
Would have been possible if the electrical gubbins was under the adjacent trailer.
In the specific case of the 3R Tadpoles, the driving trailer was from an EMU, so probably originally had 3rd rail shoes though not the rest of the kit. Had they been formed using an ex EMU driving motor coach rather than trailer, the North Downs might have had bi-modes decades ago.
No SR DEMU of any type could ever multiple with an EMU of any type; the 27 way control wires functions differed. When it was refurbled 1111 got 1966 type EMU controller in anticipation of EMU+DEMU mu operation (considered for Oxted line) but the control wires were not altered. More than one publication reported 1111 could mu with EMU but it could not; indeed it had to retain DEMU mu to work with other DEMU.
Moving Off Topic purely to cover the 210:
I wonder if the thinking behind the 210 was to develop a replacement for longer distance loco hauled services. The 90mph top speed would imply a longer distance design. Possibly the niche that was later filled by the 158. What was the highest practical power available for an engine suitable for underfloor mounting in say 1980, was this a limit of a DMU type design.
The prototype refurbished 205 in theory could, it being rewired to permit this.
As I posted upthread, no it was not.
I see, some sources suggest it was rewired.
I think they were theoretically limited to 75mph.
And would struggle to achieve even that on level track. Logs of various tours with the preserved Hastings set tend to show maxima of about 71-72mph.
11th May 1986's farewell trip produced a sprightly run above their theoretical top speed. This extract from 'Six Bells Jcn':
Most of the Southern Region DEMUs had the same power/weight ratio; i.e. 500 bhp for every three cars (or two-and-a-half cars in terms of passenger accommodation....so in theory there would have been very little difference in performance between a three car Hampshire unit with one power car and a six car Hastings unit with two power cars. Don't forget that the original batch of Hampshire units (1101-1126) were built as two car units with the same number of identical motors as the 2-HAP electric units....but with considerably more weight to lug around in the shape of a six cylinder diesel engine and generator. Also the current applied to the motors of the diesel units was limited to the capacity of the generator, whereas the HAPs could draw considerably more current from the third rail. After a couple of years in service all but four (1119-22) of the Hampshire units were given an extre centre car, which diminished their performance somewhat. Two of the remaining two car units were used on the steeply-graded Southampton-Alton service until its withdrawal in 1973....and thereafter on the much less steeply-graded Eastleigh-Fareham shuttles. The other
Back in the mid-late 1970s I've logged Hastings units doing 85 mph down Honiton and Semley banks on the Summer Saturday Brighton-Exeter service with a Salisbury driver and between Stonegate and Etchingham on a Charing Cross-Hastings fast service with a St Leonards driver. Of course this was downhill and using stopwatch/milepost timings (no GPS in those days) but I should have been accurate to within 1-2 mph. Needless to say the riding was distinctly lively and the buffet car stewards were not amused.
The use case I heard, but have no proof for, was as part of the BedPan electrification to allow extension beyond Bedford to Wellingborough, Kettering and Corby. Assume would have delivered a speeding up of MML InterCity services and a more frequent service to those Northants towns than a 90 minute InterCity service.
Happy days
My post #23 has a link to the review of May 1986's "Hastings DEMU Farewell", including a running stint "in the 80s" 
Happy days
Of course, I'm now guilty of talking well off topic and getting all nostalgic.
It's one of the 1980s tours I'd do, if I had my time again. I did 1988's "Fellsman 2" with 50024 + 50050. Vanguard threw an oil leak around Lancaster, so (me being in the front coach) I recall Shap was taken with a thick oily mist.
ISTR the Hastings sets originally had 600hp which were swapped with the Hampshire 500hp engines when the latter were increased to 3-car.
