As a former media and advertising agency person of many years, a thread like this always throws up some interesting opinions.
A lot depends on the original brief and the campaign objectives, as to what LNER are looking to achieve with this. Could be a mix of softer metrics, such as awareness of LNER, brand favourability, existing customer net promoter scores, consideration to purchase via LNER. Harder metrics could include measuring an increase in particular types of journeys, acquiring new customer segments, driving more sales via the LNER website at the expense of The Trainline and other 3rd parties.
Hard to second guess from afar, but there are lots of good reasons why they are investing in a new campaign. A lot of other TOCs invest in advertising, so they are in no way unique in doing this. As for the timing, it is the normal time of year for TOCs to be launching campaigns. It is also the time of year when the cost of media (especially TV) is often at a low point, so they will be getting more audience per £ invested.
As for the point about getting the product and service right before advertising it, I've seldom worked with any advertiser who has a product or service that is perfect. If they did, they normally wouldn't need some lovely creative work to help convince potential customers otherwise.