At the point where the rails cross (_d_, _fig. 3._) you will need a
frog, to allow your train to go smoothly over. To make this, you cut
your side rails square off at _d_, and begin it again on the inside
of the rail, leaving a space of a quarter of an inch open to let the
flange of your car wheels pass through.
Also, you must cut a notch in your regular track at the same point, so
that the wheels on trains switching off may go through (_fig. 4_).
Now your track is ready, you may begin on the train; and first the
trucks.
[Illustration: FIG. 3.]