"I don't understand why the line filled with water that is heated doesn't blow the line wide open when it gets really hot.
It's not a line; it's a boiler. When the water is heated, steam is created, which is driven to the STEAM RADIATOR.
"It's only a closed tank, which is heated to make steam;"
Water os considered to be incompressible; steam isn't. So, the unit is designed to hold the design steam pressure without exploding. So many BTU, from so much gas flow, delivered at such and such pressure, through a pipe of a given diameter, will only produce so much heat, and thus so much steam pressure.
IOW, this is all pretty easy to calculate, and the unit is very robust, with plenty of safety margin built in.
How about we go over to your house, knock you out, bring you over to Yetta's on as stretcher, and then have her revive you?
It's worked well for me, so far, although she tells me her knuckles are getting a bit sore.
If you vote for government, you have no right to complain about what government does.