There are so many things that could go wrong... I would
* check that port 55554 passes through your routing - that's one different between the two configurations. It might also be the reason you get a 502 response, if you didn't set curl to use that port. * verify that using an RFC-1918 address (10.0.0.0/8) is valid - there may be routing difficulties with that. * use netstat to verify which address/port combinations are being used a proxy ports. * use wireshark to watch the connection. I'm unclear on what network topology you are trying to construct. Do you mean you have requests that originate from eth0 and go to eth2? What addresses are associated with each interface? Do your 502 response requests connect to Traffic Server at all? My first guess would be the failing requests are using the RFC-1918 address to do outbound connections, which will fail with a 502 if the origins are not in the same network. You might need to set ip-out as well to deal with that.
