There's only so much advertising you can do. Rail definitely needs better press, I agree with you there. But it's not a substitute for investing in improvements.
If a service is little used to this day, it's likely because it's either too slow, too infrequent, doesn't go to places where people want to go, or too expensive. Usually a mix of the 4. But in my experience there are few of these lines, and if you went on the lines with the most potential (commuter lines into cities that need to be metroised, such as those into Manchester, Leeds or Bristol) you'd realise that there are actually more people wanting to use the trains than there is space on the trains for - as a result, the railway prices people off to reduce overcrowding.
The biggest reason people don't use trains more is because they're expensive. The best way to reduce fares (without serious subsidy - which is not sustainable) is to increase capacity. The Channel Tunnel's a good example of this. Avanti, LNER and Eurostar can put prices through the roof because they'll always fill their trains when they want to. If you brought fares down by running longer trains frequently, you'd no longer need to artificially raise the price of your trains to cope with the excess of passengers. Then those passengers who would use the train if it were affordable would start using it, which will raise rail's modal share by a significant few percent.
If you want more off-peak travel, then an equivalent of the Belgian Train+ (a national railcard with major discounts on off-peak fares) would go a long way.