The improvement has been incremental as it has been for decades. In fact, the biggest jump may have been with introduction of Shatabdi Express trains over last 20+ years. The one train that does impress me the most of BCT-NDLS Raj. Others are primarily reducing their slack time. Take Puri-Gandhidham Express, for example. This train had a 5:07 hour run to cover only 160 km between Nandurbar and Surat, non-stop (31 km/h). Now it has been reduced to 3:12 (50km/h). On paper, nearly two hours faster looks impressive but in reality, it is finally an average train in that section by cutting down the slack by two hours.