Switch locks
My turnout controls consist of wood dowels passing from a DPDT switch under the layout and extending through the fascia. All are oriented so that they're pushed in when the turnout's lined for the through route and pulled out to line it for diverging, exposing a portion of the dowel that's red (on mainline turnouts) or yellow (all others) so operators can tell at a glance if any turnouts in a town are lined against them.
Because the controls are pulled out to line the turnout for the diverging route, simply drilling a hole in the dowel to allow a small padlock shank to pass through wouldn't work. Also, because many of the dowels pass through the fascia at an angle, or at a point where the fascia is curved, adding some sort of collar around the opening to enable locking wouldn't work either. However, a recent trip to a local home improvement store to scout out hardware options resulted in what I feel is the perfect solution: Small screw eyes such as those shown below, $0.69/package.
With one screw eye passing horizontally through the dowel aligned with another that's screwed into the fascia, a small luggage padlock can be inserted, preventing the turnout from being thrown. I found these padlocks on Amazon for $9.95 for a package of 10.
Here's a pic of everything in place for the West Siding Switch (labeled "WSS") at Hillis:
Note that the padlock isn't actually locked. The shank is simply aligned with the hole in the body to give the appearance of being locked. This idea was what really sold me on the use of switch locks, as it makes them much less cumbersome when operating solo, and also keeps crews from having to fumble with keys, forgetting to return them after a session, etc. Just hanging the locks in place, unlocked, still provides the operator the feeling of taking that extra step before a turnout can be thrown, without the hassle of putting down my throttle, paperwork, uncoupling pick, etc. every time when I'm operating alone. (Also note that I later changed my mind on this approach after finding that it was actually fun to add the activity of unlocking and locking the switches.)
To line the turnout for Hillis siding, you just swing the padlock open and lift it out...
...then pull the turnout control to line the switch, hanging the lock on the fascia until the turnout is restored.
Also note that, just like on the prototype, only the mainline turnouts outside of yard limits get Transportation Dept. locks used by train crews. The Pocket track here on the left (labeled "PKT") diverges from a secondary track, so no lock is necessary. For my entire 125' mainline, the only switch locks that train crews generally have to think about are seven that have Transportation locks.
So that covers the locking of turnouts through which train crew will generally operate. However, the final reason that I mentioned above for switch locks was to "prevent crews from operating on trackage that, on the 1:1 IAIS, is controlled by the Engineering (MOW) or Mechanical departments (diesel house/carmen)". This keeps my HO scale crews from becoming confused and using tracks where, for example, MOW ties down their equipment, or where the diesel or car shop crews are performing service or repairs. On the IAIS, switch locks for the Engineering Dept. carry yellow tags, while those for Mechanical carry blue. Here's a James McNab photo of an IAIS Engineering Dept. tag on the Grimes Line's Pharma Tech spur:
I recreated those for the layout using card stock from Hobby Lobby, cut to 11/16" wide by 1-1/2" long. Here's an Engineering Dept. lock on the McClelland Elevator spur, used exclusively by MOW now. Note that, unlike the Transportation Dept. locks controlled by train crews mentioned previously, the Engineering and Mechanical locks on my layout are truly locked. If crews encounter a need to access one of those tracks (e.g. moving MOW equipment, spotting to the RIP, etc.), they have to contact the Roadmaster or enginehouse foreman, respectively, to be let in.
Finally, here's a shot showing Mechanical Dept. locks (blue tags) on the Runaround and Back Lead switches at Council Bluffs, protecting two of the three entry points into tracks controlled by enginehouse and car shop crews. The third is out of view in the distance.