The Tadpoles (3R, or class 206) were very inefficient, having only 72 seats in a three car unit - the 66 seats in the driving trailer (DTS) were virtually never available as the coach was always locked OOU.
The "Tadpoles" were introduced c.1965 and operated an hourly (almost) all-stations Tonbridge - Reading. This was a response by Southern to the Beeching proposal to withdraw all stopping passenger trains: the "basic" service on the Paytrain principle, with most stations de-staffed and buildings demolished, and guards issuing tickets, most likely averted this and saved the service.
For a few years in the late 1970s I commuted between Reading and Redhill. The Tadpoles were certainly "interesting"! Whether the driving trailer was unlocked or not, when required, seemed to depend on whether the guard could be bothered; I would say about 50/50. There were (probably still are) heavy school movements between Reigate and Deepdene, and Crowthorne and Wokingham which totally wedged out the 2 cars if the third was not available. The DTSs were almost always locked out off-peak, as they were compartment seats with no corridor connection to the other two cars, so fares could not be collected. As
@Gloster says above, other DEMU types were rare at this time, so the Tadpoles must have been pretty reliable. Several peak extras were Class 33 hauled, which must have been inefficient. The morning school train from Redhill to Tonbridge was a 6-car Hastings DEMU which continued to Hastings.
Their replacement by first-generation WR DMUs was a substantial improvement.
Well - the first lot (117s? suburban units) were only better in offering more seats. With doors to every bay they were draughty, seats were basic bus, and they also struggled to keep to the timings designed for DEMUs. (Of course to Tonbridge and Redhill drivers they were quite alien!) The later Met-Cams (101) were better and more comfortable and the Cross-Countrys (119? - sorry, not good on class nos.) were nicer all round.
There was, I recall, a year or two in the (?) early 90s when the line came under the South Western division who seemed to have no interest in it (nasty old diesels!) and service quality went downhill. They also slashed the Redhill - Tonbridge line to 2-hourly, even in the peak, and lost a lot of school traffic, some never recovered: to this day there are still 3 school buses each day between South Nutfield and Redhill schools that had to be provided at short notice.
With the move over to the Western (Redhill - Tonbridge went to South Eastern, then was electrified around 1993, ostensibly for Chunnel freights), and then the semi-fast Reading - Gatwicks the situation improved and traffic started to recover.
Hope all this piffle is of some interest - please correct any errors of memory - they were certainly interesting times for me and almost made my boring job worthwhile!