I suppose this depends on what people think the railways are for.
Some people take the view that railways should be a means to an end intended to be as efficient as possible (with some kind of definable objective means of assessing it), e.g. to remove the most car miles up to a certain level of subsidy...
...or whether the railway is an end in itself and is something that everyone should have local access to (regardless of whether the population density/ demand justifies it). And some people seem to have a funny idea about distribution of railway stations - it's apparently important to link a rural settlement with ten thousand people but people turn blind eyes to large urban areas where the nearest station is in the city centre (hence the obsession with places like Devon villages rather than, say, fifty thousand people living in Leith).
I believe that heavy rail is only one tool (and that it's a pretty blunt/ inflexible tool, best suited to the bulkiest flows). Brilliant at what it does best but it's one end of a toolbox that includes trams/ tram-trains/ coaches/ buses/ guided busways/ dial-a-ride minibuses.
Looking at the OP's example...
There are, as you'll be aware, many towns in the UK that are not served by railways. Leigh for example, Gosport, Skelmersdale, Washington, Ashington, Portishead, Clevedon as others
...Washington is the place that I know best - it's certainly a large place to be without a station but it's a difficult place to serve easily given how spread out the housing estates are - one station is still going to be a long way from most people - presumably it'd be on the Leamside line, so on the western fringes of the town - some distance from the Galleries and from Concord - requiring a bus or drive to the station (and Washington residents already have that facility with the Go Ahead 4 to Heworth Metro Station, as well as regular buses directly into central Newcastle).
If opening a "stub" branch into Newcastle then you'd need to offer a very attractive train frequency to get people to head over to the far side of Washington! But if you are opening the Leamside line throughout then it makes sense to put a station in at Washington rather than omit it.
Maybe a Metro extension from South Hylton through Washington to Gateshead would work, since that'd be at lower cost and would provide links to other parts of Wearside (but then you have the problem with that getting in the way of re-opening the Leamside to heavy rail at a future date - would people be happy with this, if it meant that re-opening the Leamside throughout meant paying to upgrade the Metro to Tram-Train?
Ashington and Portishead, on the other hand, are both places with much denser populations so more suited to heavy rail (given that re-opening the old train line will be pretty close to the town centre and also within a reasonable walk of people's houses).
Gosport... tricky one - what train services would run there? You'd have the problem that any medium/long service to Gosport would be at the expense of serving Portsmouth/ Southampton ... and if you are only talking about a shuttle service to Fareham then that doesn't offer much that the frequent bus service doesn't.
Skelmersdale would probably be top of my list since there's an obvious train service that you could extend there, a train service that runs to the destination that would probably be number one for residents - I'm surprised that it wasn't done decades ago.
But I'm not sure whether the New Town stuff on here (e.g. there were deliberate plans to stop people returning to the city they previously lived in) is true or not - as far as I see it, the new towns were being built at a time when rail was out of favour (rather than any conspiracy)