That's a cute trick, but to me that's all it is. I feel like the version kikopup has her dog do is just impractical in a public place like a train and really is just too much. I prefer to teach the much simpler version. Because I'm too lazy to type it out I have found a lovely explanation from another service dog user of how she taught it.
"Make her use her brain to figure out what you want her to do. To do this, have her lay down and lightly press against the side of her tail with your foot. As soon as she moves her tail on her own (you must wait for her to move her tail on her own!) click and treat. Repeat many times.
At first, she probably will not know what you are clicking for-- her movement of her tail may be involuntary. But you will see her try to figure out what it is you are clicking for. Depending on how aware she is of her tail muscles, she will eventually figure out you are clicking and treating for movement of the tail. Once she figures this out, you can start waiting to click and treat for the tail movement you like-- so while before you were clicking for any movement of her tail, start slowly requiring more of her.
At first require any voluntary movement of her tail. Then require the voluntary movement to be 1 cm, then after a few
training sessions require it to be 2 cm, then 3 cm etc until you've got her tucking her tail all the way under. At this point and only at this point should you introduce the verbal command. Do not introduce the verbal command earlier than this as it will only confuse her about what you are asking for.
To introduce the verbal command, say the command, then give the pressure on the side of her tail with your foot. Repeat a bunch of times. When it seems like she is starting to anticipate your foot pressure when you say the verbal command, start giving some time, like a second, between the verbal command and the physical stimulus. This will give her time to think. Eventually she'll not need the physical stimulus and she'll understand that just the verbal command means to tuck the tail."