29 you have to make sure that there is only one place for the cart to teleport to and the track is not allowed to be on the same height
for example if t=track and d=some block and o=bottom obsidian of portal then is this incorrect
ot
dd
and this correct
o
dt
you also need to make sure that either the cart wait a certain amount of time before it re enters the portal or that it gets replaced (that is what I choose to do)