The rails of a trussed beam are separated by __________.

The rails of a trussed beam are separated by truss blocks, which play a crucial role in maintaining the structural integrity and alignment of the trusses. Truss blocks are specifically designed pieces that provide a fixed distance between the vertical members and the top and bottom chords of the trussed beam, thereby ensuring the design maintains its intended geometric configuration. This separation is essential for the truss to function properly as it distributes loads and helps prevent buckling or deformation under stress.

The other options do not fulfill this specific role. Supports are typically external structures that provide stability but do not separate the rails of a trussed beam. Braces are used to strengthen the structure by providing lateral support but are not responsible for separating the rails. Connections refer to the joints or fittings that hold various components of the truss together rather than separating the rails themselves.