At least four times since 2015, I've posted the idea of a working remote-control brake on an observation car or caboose.
Ted's posts about a working train brake
The CAB-2 (R.I.P. ) has a train brake slider. Why not make it actually do something? If it weren't for the &*%&# rubber tires, you could apply the brake and come to a stop stretching the slack until you get a little wheelspin! I suggested the design could be a solenoid that pulls up to spread a pantograph with the sides rubbing on the wheels. We're thinking alike here.
I like geysergazer's solution because it's simple. From an implementation standpoint, I'm afraid it won't be practical or cost-effective to retrofit large numbers of cars with working brakes. Or body-mounted Kadee couplers such as the 3-rail scalers use. Those might work on your O72 timesaver, but definitely not an option for sharp toy train curves.
I smell a new thread topic coming soon. Caboose with DIY Working Break.
At least they are using that train break slider to do cool things like activate wheel slip on the triplex, and it triggers the sound effect of wheel slip on the big boy. Also, good news is that even the CAB3 App has a train brake slider on it.
And something like that pantogaph idea is certainly more complicated to implement, but maybe it could be done well with nothing but laser cut parts I am envisioning something like a car scissor jack design, the advantage to this scissor/pantograph idea is that you could theoretically get both axles on the same truck at once. Clearance might be an issue, though, since I just remembered that the bulk of the break will have to exist on the truck to account for the rotation independent of the chassis. Maybe a model air plain push rod cable could do the trick for us here. Heck, with a push cable driven by a servo, you can get both the front and rear trucks at once. Maybe we just pull an actual rubber brake shoe with that cable. I think we just invented the train break, lol. A 9 gram servo as the actuator and we got something here.