Estimate train ticket costs.
Rail spend is the number of single journeys times what each one actually costs you, which is the headline fare less any railcard discount plus any per-ticket booking fee. Counting in singles rather than returns avoids the ambiguity of return tickets that are sometimes barely dearer than a single. The saving line prices the discount over the whole travel period, which is how you judge whether a railcard's annual fee pays for itself.
Effective fare
Effective fare = standard fare x (1 - discount) + booking fee
Total
Total = effective fare x journeys per week x weeks
Usually somewhere between three and four return trips a week on the same route, though the exact break-even depends on operator pricing. Work out the season price per journey at your actual travel frequency and compare it to this effective fare.
Often, on longer routes where fare boundaries create anomalies, with savings of 20 to 50 percent. The tickets must cover the whole journey and the train must call at each split point.