I've done a lot of research in to the market for heritage railway through customers from the mainline. The market is surprisingly small and largely depends on the specific geographical location of the interchange station and it's proximity to metropolitan and tourist areas. The fact that the Swanage Railway is subsidising the Wareham link by in excess of £100k pa says it all really.
I have to confess to being somewhat intrigued by this "subsidy" ie a loss of £100K.
My intrigue stems from:
1) the DMUs (1 X Class 117 and 1 X Class 121) were provided free of charge by Dorset County Council who paid the vast majority of the cost of refurbishment to mainline standards. The shortfall was caused by the cost of the refurbishment increasing because
all the axles and wheels on the units were condemned when crack tested. SRC had not taken account of this possibility. All the other capital costs (line upgrade and signalling improvements) were paid for by others. (Chalk and Cheese Fund and Network Rail).
2) The DMU servicing centre at Corfe Castle Station was funded (whether 100% or less I do not know) by a grant from the Talbot Trust.
3) Despite many years work (at least 8) and the payment of fees to Rail Business Solutions, SRC has not been granted its mainline operating certificate (or whatever the correct wording is). The service is being operated by WCR using WCR drivers and SRC Guards and TTIs (I believe) but using the aforementioned SRC rolling stock.
4) The cost of Public Liability Insurance for operation on the mainline is very high indeed even though the amount (distance) of running on the mainline is small. (From Worgret Junction to Wareham Station). I think but am not sure that WCR's insurance has to be used.
5) SRC does not set aside money from revenues for future refurbishment of rolling stock. IE there is no fund for the DMUs overhaul when they become due.
6) It also appears that the Wareham service is being run as some form of pseudo charter so that some of the passenger service obligations of running a scheduled servcie do not have to be complied with.
If the loss ends up being £100K over 90 days service (compared with £70K over 60 days service in 2017) then proportionately it is slightly better but in 2023 SRC does not have to hire in the rolling stock it did in 2017. Looking at the two years, despite the difference in economic circumstances, the result is still the same - a significant loss. As is said in the film Frozen "let it go".
IMHO SRC should tell Dorset Council that the Wareham service is unviable and say it will not run again. If Dorset Council want SRC to operate it, then Dorset Council would have to pay at least twice Swanage Railways loss on this years operation (if not more). The reason I say twice is that I am not confident that Swanage Railway allocate all the costs incurred by an activity and SRC could not afford to run the risk of undercharging Dorset Council and finding itself in financial difficulty as a result of operating the service in future years.
I struggle to see why potential customers would drive to Wareham (and park) to catch the DMU service (whether to interchange or not at Corfe Castle to steam) when they can drive to Norden and catch the steam service from there.
The benefit is in persuading customers to ditch the car and use mainline services to Wareham and connect there. However imho it just is not sufficiently appealing for customers to get mainline to Wareham and connect for services to Swanage: combined fares too high, service only operates 4 days a week, lack of frequency, concerns over connections etc.