Changing the last line to: 
 
 s1 = NDSolve[{equa00, x[0] == 1, x'[0] == 1}, x, {t, 0, 50}, SolveDelayed -> True]

or

 s1 = NDSolve[{equa00, x[0] == 1, x'[0] == 1}, x, {t, 0, 50}, 
 Method -> {"EquationSimplification" -> "Residual"}]

seems help for your problem.

**In reponse to updated question on plot slution**

To plot your solution, maybe this is what you want?

 Remove["Global`*"] // Quiet;
 m = 100;
 a = D[x[t], {t, 2}];
 t1up = 2 x''[t] + 1/2 (490 + 34 x''[t] + 2 (490 + 50 x''[t]));
 t1down = 490 + 53 x''[t];
 t1 = Piecewise[{{t1up, x'[t] >= 0}, {t1down, x'[t] < 0}}]
 equa00 = t1 == m*a
 t0 = 50;
 s1 = NDSolveValue[{equa00 // Simplify`PWToUnitStep, x[0] == 1, 
 x'[0] == 1}, x, {t, 0, 50}];
 sAll = {x[t] -> s1[t], x'[t] -> s1'[t], x''[t] -> s1''[t]};
 
 t1upvalue = t1up /. sAll;
 t1downvalue = t1down /. sAll;
 t1value = 
 Piecewise[{{t1upvalue, s1'[t] >= 0}, {t1downvalue, s1'[t] < 0}}]
 Plot[t1value, {t, 0, t0}, PlotRange -> All]