My understanding was that the plan to extend certain Cambridge services out to 12 cars in length pre-dated any plans to introduce trains capable of selective door opening (SDO) onto the routes. Therefore there would be a need for platform lengthening at certain locations to accommodate these services. (As an aside, it is always better to platform the entire train rather than rely on SDO as it speeds up loading and unloading and therefore reduces station dwell times.)
At the time, the stopping pattern for the semi-fast services to Cambridge were Tottenham Hale, Cheshunt, Broxbourne, Harlow Town, Sawbridgeworth, Bishops Stortford, Audley End, Whittlesford Parkway and Cambridge. Of these, all but Cheshunt, Broxbourne and Sawbridgeworth were already capable of accommodating 12 car trains, so these three station were targeted for platform extensions to permit the existing service pattern to be maintained. Stansted Mountfitchet was additionally earmarked for a platform extension due to the original plan of running 12 car Stansted Express services, as many of these services are timetabled to stop there.
What actually happened was, as Bald Rick says, the demand for a 12 car Stansted Express service disappeared and the balance of the 30 unit Cl379 order not used on Stansted Express services were used instead on Cambridge services. Whether or not this was instrumental to the Cl317/7 units going off-lease is open to debate, but the fact of the matter was that having placed the order for these new trains a use had to be found for them and the Cl317/7s were somewhat non-standard and, therefore, ripe for withdrawal.
What was not initially foreseen was that these 12 car Cambridge services would be express in nature and, therefore, would not call at those stations with newly lengthened platforms designed for just such services as these (nor those south of Audley End that could already accommodate a 12 car train). While I was never directly involved in operating these trains and was never particularly familiar with the peak time services from Cambridge, my understanding was always that these 12 car express services were run in addition to those services operating from Cambridge and, therefore, effectively took the pressure off slower services calling at places such as Sawbridgeworth.
If it helps, what I often observed is that passengers would join the Stratford service at Sawbridgeworth and then change at Harlow Town onto the Stansted Express service that follows this slower service, allowing them to get to Tottenham Hale and Liverpool Street sooner than waiting for a direct train. But to answer the original question, the service pattern always appeared to be about right for the location. Given the numbers that I saw travelling from Sawbridgeworth, 3 trains per hour off peak was about right and the peak time frequency with the additional stops on Stansted Express services, etc, seemed adequate. Compared to stations such as Ware on the Hertford East branch where the number of passengers always seemed to be much higher and with it's off peak frequency of 2 trains per hour and some additional services in the peak, I would argue that Sawbridgeworth doesn't do so badly.
However, what Sawbridgeworth suffers with is being that much further down the line past places where the services load up pretty heavily, such as Audley End and Bishops Stortford. Does this merit the introduction of a stop for the 12 car services? Perhaps. I always wondered about the loading of these trains given their stopping patterns but, having never worked them myself (they weren't operated by drivers from my depot), I can't really comment. Either way, the provision of an additional service stopping elsewhere can help to alleviate the problems of overloading that would otherwise exist. That is why some Stansted Express service stop (stopped...?) at places like Enfield Lock and Brimsdown during the morning peak. If they didn't, the loadings on the services coming up from Hertford East directly behind them would be impossible.
O L Leigh
