The plot shows the position (x) as a function of time (t) for two trains that run on parallel track. Train A is next to train B at t = 0 and 

At ${\mathrm{T}}_0 \mathrm{both} \mathrm{trains} \mathrm{have} \mathrm{the} \mathrm{same} \mathrm{velocity}$

Both trains speed up all the times

Both trains have the same velocity at some time before ${\mathrm{T}}_0$

At ${\mathrm{T}}_0$ the trains have covered different distances