a = t;
b = 1;

a/.t->Range[1,5] (*returns: {1,2,3,4,5} *)
b/.t->Range[1,5] (*returns: 1           *)

{1, 1, 1, 1, 1}

express = t^2; (* OR t, OR 1, OR 3*t+t^2 *)

tSet = Range[1,5]
express/.t->tSet

express = 1;
express/.t->tSet (* will output just 1, I would like it to be list of ones of length 5 to match tSet *)