Concatenation

The most basic operation is concatenation, for which the operator is invisible. It works exactly like you think. Entering x y z produces
\begin{rail} x y z \end{rail}
Related to concatenation is the empty body. It can occur only as operand of |, * or + or within parentheses. A safe way to write the empty body is (), which displays as
\begin{rail} () \end{rail}
i.e., nothing. However, the invisible empty body is useful with the repetition operators, as will be seen below.

A feature related to concatenation is the ability to split long horizontal sequences with \\. As an example, a b c d e \\ f g h i j results in
\begin{rail} a b c d e \\ f g h i j \end{rail}
It is possible to create horrible graphic constructions with this operator, for example \\ | \\:
\begin{rail} \\ Vert \\ \end{rail}
or ( \\ * \\ ) \\:
\begin{rail} ( \\ * \\ ) \\ \end{rail}
Nevertheless, every input should result in reasonable output.