in Train Simulator Classic itself. The game only automatically updates core DLC purchased through Steam. For Indian add-ons (many are third-party, not on Steam), you have to manage updates manually.
A 165 km stretch from Mangaluru to Murudeshwar available at Indian Train Sim. train simulator classic indian addons upd
Launch TS Classic → click Settings → Tools → Refresh Blueprints Cache (always after manual install). in Train Simulator Classic itself