It's 80 kmph. :)
I should have been a bit more clear about what I meant though. Beyond 80 kmph, the effect of the coach's cross sectional area becomes a significant portion of the total resistance to be overcome. It does not make a big difference to the ride quality or hamper the train in achieving a higher speed, but the locomotive has to put in more energy to overcome the resistance. But yes, it is limit only as far as energy efficiency is concerned, not ride quality or feasibility of high speed runs.
...
more... If the air resistance from head-on-wind is considered, then depending on the specific conditions, sometimes, normal non-streamlined locos ( like my favorite WAM4 :P ) or coaches with high ground clearance, can also move at speeds of 200kmph +. This is because, the fast moving locomotive compresses the air in front of it ( similar to a falling meteorite, where this compression is called ram pressure). This compressed cone of air usually acts as a streamlined face, reducing the air resistance by a good amount. But for the case where the winds are hitting the train at an angle, directly hitting the side walls of the coaches, the coach ground clearance, height etc becomes an issue and this truly restricts the normal operational speeds and this will affect ride quality as well.