Bidirectional without backing up. To turn a train around, you need a closed loop at the end of the line or a triangle (wye).

One of the things that makes it more interesting in practice is that most electric model trains* use a "one rail is hot, one is return" electrical design-- so adding a wye or reversing loop to the design is asking for all sorts of electrical gimmicry to prevent it from shorting, while still maintaining usability.

*classic Lionel and Marklin designs used a third conductor in the centre. Since the outer rails are wired together, reversing designs are trivial.