I just wanted to run something by you guys. Most of you here have alot more knowledge about train horns and their function than I do. I know lanyards are common on big trucks, but could it be used to blow a train horn like a K5? I was thinking about going that route “if” it’s possible. What’s your take on this?
I’m not too familiar with lanyards personally. My manager said that they may be to restrictive for the Nathan AirChimes. Maybe some of these guys know more.
Wolo I believe makes a lanyard style air valve, but as mentioned it will probably be very restrictive for any bigger style air horn. There is a style that we have in one of our fire apparatus, it’s a lanyard “switch” type. You mount it, attach your lanyard, and run wires to a regular air valve like HB sells. This may be your best bet. This option also keeps you from having to run air lines through your vehicle roof and door pillars.
Best of luck to you
Update:
Below is what I was talking about, the first one just search for wolo pull valve, probably doesn’t have any bigger than 1/4" opening for the air though so it will really limit a train horn. The second one may work better cause you can hook it up to any air valve you want to. It’s normally open, pull to close (normally off, pull to activate) and the hole on the end looks like you could fish some wire or rope through to act as your lanyard. The link is for the company that sells the switch. Hope all this helps!!
Forget about truck air horn valves when you have a K5. They just won’t flow the required air. Look into getting a 1/2" manual valve like a Graham White 353 or clone and rig it up with a pulley & lanyard. User Marlin009 rigged up something like that and it seemed to work OK for him:
http://www.trainhornforums.com/showthread.php?t=6092
or maybe one of these! 
http://www.ebay.com/itm/40100-COLE-HERSEE-SIGNAL-BUZZER-CONTROL-SWITCH-/300788213089?pt=Vintage_Car_Truck_Parts_Accessories&vxp=mtr&hash=item46085fe561
Here are the two manual valves that we have. There is also a thread about the valves on here.
http://www.hornblasters.com/products/details.php?i=graham-white-353-locomotive-hand-valve
http://www.hornblasters.com/products/details.php?i=half-inch-manual-train-horn-valve