it is not possible to plot it in higher Mathematica versions
I bet it's possible if you wait for enough time. You don't see the result mainly because v5 is using very few points for the generation of graphic:
plot = Plot[T, {τ, 0, 6*10^3}]; // AbsoluteTiming
(* {32.1406250 Second, Null} *)
plot[[1, 1, 1, 1]]
% // Length
To obtain the result in a reasonable time, just limit the points for plotting:
Quiet@Plot[T, {τ, 0, 6*10^3}, MaxRecursion -> 0] // AbsoluteTiming
Still a bit slower than v5, but acceptable in my view.