Here is a schematic of the Triac schematic I've used for all my sidings. It's important to use it with a thermal circuit breaker in series with the "hot" (non-common) output of your transformer or TIU for protection of the Triac. (It's always good practice to use a circuit breaker for protection of all your electronics.) Note that you need to use switch contacts that are built into many turnout switch motors. You can also do siding block control with a SPDT mechanical relay but that typically requires a separate fixed power source to energize the relay. I can also provide you a schematic of that approach. The advantage of the solid state approach is that no separate power source is needed but the minor disadvantage is that you get a voltage drop/loss of about 1 volt AC (i.e., an output of 18 VAC from the transformer results in about 17 VAC delivered to the track.) This has never caused a problem on the sidings using conventional, DCS or Legacy/TMCC operation.)
For people using the DZ series switch machines, a DZ-1008 Relay Module can simply be used to apply power to the siding making things very easy!