[tacticalogic]

But the key point is Congress may regulate intrastate rates if they affect the interstate rates that Congress is regulating.

Congress did not regulate the intrastate rates. They regulated the interstate rates. The corresponding affect on intrastate rates was do to the way the rate schedule was contrived by the railroad. The could have kept the intrastate rates the same in spite of the regulations being imposed on the interstate rates. They simply chose not to.

[offwhite]

"The could have kept the intrastate rates the same in spite of the regulations being imposed on the interstate rates. They simply chose not to."

You say you read this case?

Some carriers were inflating their interstate rates to offset their deflated intrastate rates, making intrastate-only carriers non-competitive. Since there was a direct relationship between the rates, Congress needed to regulate both.