Alternate
When your scooter has a fault, you need to get an alternate. Hopefully your insurance will cover it too.
You have a scooter that you drive every single day, but if you notice a fault, you need to send it in to get it fixed. That's when you break out your alternate scooter.
てつ
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The alternate scooter is made entirely of 鉄 (てつ). It's hard and cold and not very comfortable when you sit on it. That's why you chose to keep it as an alternate and not as your everyday scooter.
Imagine riding on your alternate 鉄 scooter. Feel the hard iron under your bum and the loud sounds it makes as you scoot down the street. I hope your normal scooter is fixed soon.