As of October 2026, the largest source of tax revenue of the Union Government of India is—

Direct taxes, mainly personal income tax and corporation tax, now bring in well over half of the Union Government's gross tax revenue, more than GST or Union excise duties. So the answer is "Direct Tax".

Union excise duty was the largest single source in earlier decades, but direct taxes overtook it in the mid-2000s, and since GST (2017) excise is levied mainly on petroleum products and tobacco. Sales tax was a State tax, now largely replaced by GST, and Railway receipts are commercial earnings of the Railways, not the main source of revenue.