The 08.30 SX Southend Vic to Liverpool St is another train that uses the up I line round the side of the flyover. Off the top I can't think of any booked passenger trains on the down I line.
On the up main the route is signalled for 50mph with flashing yellows after passing Seven Kings (not that common on the GEML). Crossing back to the up electric line at the foot of the flyover before Manor Park is also 50mph. The rare track for passenger services is to continue round the back of Manor Park station and join the up electric at Forest Gate Jn, but that is a slower route.
In the other direction it is much slower, and is most commonly used by ECS after the morning peak going to Ilford Car Sheds. They are handy routes for crossing main to electric without conflicting with traffic in the opposite direction.