Brazzaville–Kinshasa Bridge

The Brazzaville–Kinshasa Bridge is a planned road-rail bridge that will connect Brazzaville in the Republic of the Congo and Kinshasa in the Democratic Republic of the Congo, two major capital cities on the Congo River.^[1]^[2] The project has proceeded intermittently, but work is slated to begin in August 2020 after receiving a $210m loan from the African Development Bank.^[3]

Advantages

Amongst other things, the proposed bridge would connect Kinshasa via rail to the deepwater 10.46 m draft port at Pointe Noire^[4] rather than the shallower 8.20 m draft river port at Matadi.^[5]
The railway systems of both countries use the same gauge, namely 1,067 mm (3 ft 6 in)
With ever bigger ships, shipping companies prefer to use fewer ports.
The connected railways can share rolling stock which saves capital costs.
If one line is closed, for example by floods, traffic can be diverted to the other line.
There are potential lines from Angola and Namibia which would create a better network.
It will allow people to drive directly from South Africa to Eastern Russia, becoming part of the longest driven road possible.

A minor detail would be to eventually unify couplers and brakes, etc., if not already compatible.
The Republic of the Congo might be more exposed to political turmoil from the Democratic Republic of Congo.
The great expense of construction.
Cost of maintenance would be an ongoing stress on both countries' budgets.