This does not seem to make much sense. Spacetime is not a medium in the sense of a substance whose oscillations enable the propagation of waves. EM waves don't need a medium.
By "limit", perhaps you mean "speed limit"? If you do, then you need to qualify what you mean, because certain phenomena are not precluded from moving faster than c, for example the phase velocity of light under certain circumstances.
Yes, this is why both of you are wrong about this, you haven't taken in account the Energy-mass curvature of time-space or gravity of either of these objects, Yes, it is a medium that effects all of this stuff otherwise, why are black-hole's black if light was not attracted via Time-space Curvature or "Gravity" they would not be black. Yes, it makes a huge difference. In all of this.
This is the same reason, I called http://www.sciencefo.../86228-strange/ literally retarded for not getting this concept and having the same misconception.
This is going to use a shitty Newtonian Gravity Equation, so you will understand it, but is still valid.
M'Train = MTrain rest/(1-(VTrain2/C2))1/2
L' Train= LTrain rest(1-(VTrain2/C2))1/2 ,
Δt'Train = ΔtTrain Rest/(1-(VTrain2/C2))1/2
Vgravity Train = (GM'Train/L'Train2)Δt'Train
Vgravity platform = (GMplatform rest/LPlatform rest2)Δtplatform rest
∑Vtrain = Cphoton + Vgravity Train - Vgravity Platform - VTrain
∑Vplatform = Cphoton - Vgravity Train + Vgravity platform
Then the train moving so
∑VTrain Δt'Train = ΔxTrain
Then the platform is at rest so
∑Vplatform Δtplatform rest = Δxplatform
Edited by Vmedvil, 10 January 2018 - 01:18 PM.