There are many conundra with light rail (tram-train) operation on an existing heavy rail line, and it all comes down to aligning the two safety cases.
If the light rail vehicle already operates on line-of-sight (i.e. no signals at all) then it doesn't actually matter what the rail vehicle ahead is, because our light rail vehicle is already approved. The other way round is more complex, but TPWS is the answer. TPWS could be fitted to every signal, effectively as a train stop, and appropriate controls applied that maintain a safe distance between our 2000t freight and the tram ahead.
The only issues are allowing line-of sight operation on Network Rail (or indeed any Railway, as distinct from a Light Railway or Tramway, as legally defined) and how you detect a vehicle that isn't designed to operate track circuits.
One could fit the light rail vehicle with TPWS, but low-floor trams don't have much spare space underneath and managing the activation and isolation of it introduces risks).
All that said, Mott MacDonald (who are currently employed to resolve these issues) must be getting there, as the DfT has committed to it.