There are many reasons for the higher speed on KR as compared to ST-BSL
(1) ST-BSL track layout was fixed about a century ago, for MG route when the max. operational speed of trains of BG route was just 60kmph. It was in-fact 67kmph till the first wave of MG SF trains Pink City, Vaigai, Ashram etc with 90/100 kmph were introduced in late 1970s. More over rudimentary tunneling & bridge technology available at that time led to relatively poorer track design/ geometry.
(2) KR way laid in 1990s for BG train...
more... operation and designed in view of relatively much better construction technology with a speed potential of 160kmph. As a result the track layout is much better, inspite of Konkan terrain being much tougher than Tapti valley.
(3) Credit goes to Mr. Sreedharan who did not compromise on the quality aspect and went for Optical fiber communication (costing around 55 crores at that time) and modern signalling etc. inspite of stiff opposition from some quarters of Rly Ministry.
(4) Mr Sreedharan however,later lamented that, due to extreme shortage of funds he could not go after better soil/subway/hill stablisation measures to his choice, else the incidences of boulder falling would not have happened and the need of monsoon time table i.e. slower running of trains during monsoon might not have arose.
(5) I always feel that IR must go after a long term (10-15yrs) track upgradation plan for total IR network, to improve the track layout, and strengthen the track, and modernise it with latest state of the art signalling & control systems. Such measure shall ensure higher operational speeds and much higher avg speed throughout IR network in due course of time.
(6) Tapti line is under brisk doubling as it have significant freight as well as respectable passenger traffic (Guj/Raj/Maha bound). Avg speed of trains is increasing and shall soon increase till doubling is completed in next 1-2 years.
(7) But Tapti line probably does not the same importance as Konkan Rly, from national perspective. Konkan Rly (+NDLS-BCT line) is a promising North-South connectivity and may become competetor of NDLS-MAS-TVC trunk route after doubling and electrification. On the other hand tapti line is a second fiddle to main East-West connectivity i.e. CSTM-BSL