See here:

https://community.wolfram.com/groups/-/m/t/440741

Edit:

I have a solution of myself:

Sum[UnitBox[k (x - T j)], {j, -10, 10}]

UnitBox gives a top hat centered at 0, width 1. The k makes it with a shorter width. The T is the periodicity.

When k=1/T, you get a straight line.

And when k<1/T you get a different thing than the one you asked for.

But if k>1/T, you get this pulse train.

You can also add a phase, to make the function start where you want:

Sum[UnitBox[k (x - T j+phi)], {j, -10, 10}]

With a phi that runs from 0 to T.

Change the values for j to get the number of pulses. Number of pulses=2j+1.

I hope it helps!