I have a cast iron sewer main in an older home. Just where it exits from under the house, it takes a 90 degree turn downward, then 180 back up, forming a trap. I've never heard of a trap in the main line. Is this normal? Will it lead to clogs? Seems that solids would tend to accumulate at the bottom of the "horse-shoe". Should I eliminate it. The main is about 6 inch diameter, but it is reduced to about 4 in. where it is coupled to PVC leading to street.

The only reason I can think of this originally being fitted is that on its return up it doesn't reach the same height as the house end therefore preventing water entering the house through old inadequate sewerage systems in the street and being flushed out by the top feed of water leaving the house therefore not trapping anything as such if this is not the case and they both are at the same level eitherside of the bend then I see no harm in replacing it but then again " if it aint broke dont fix it "

I have a cast iron sewer main in an older home. Just where it exits from under the house, it takes a 90 degree turn downward, then 180 back up, forming a trap. I've never heard of a trap in the main line. Is this normal? Will it lead to clogs? Seems that solids would tend to accumulate at the bottom of the "horse-shoe". Should I eliminate it. The main is about 6 inch diameter, but it is reduced to about 4 in. where it is coupled to PVC leading to street.
House traps were used years ago. They are not permited by today's plumbing codes. As long as it works, your OK.

I have a cast iron sewer main in an older home. Just where it exits from under the house, it takes a 90 degree turn downward, then 180 back up, forming a trap. I've never heard of a trap in the main line. Is this normal? Will it lead to clogs? Seems that solids would tend to accumulate at the bottom of the "horse-shoe". Should I eliminate it. The main is about 6 inch diameter, but it is reduced to about 4 in. where it is coupled to PVC leading to street.
There was a time when whole house traps were the norm.

If it were me, I'd remove the running trap, especially if you've upgraded any of your plumbing fixtures in the past decade -- The lower consumption fixtures tend to lack the velocity to push solids through a running trap.