Two parallel rail tracks run north-south. Train A moves north with a speed of $54\text{\hspace{0.17em}k\hspace{0.17em}}m{h}^{-1}$ and train B moves south with a speed of $90km{h}^{-1}.$ The relative velocity of B w.r.t. A is

Let positive direction of motion be from south to north.
Given, $v_A=+54km{h}^{-1}=15m{s}^{-1};$

$v_B=-90km{h}^{-1}=25m{s}^{-1}$

$\therefore$ The relative velocity of B w.r.t. A

$v_{BA}=v_B-v_A=-25-15=-40m{s}^{-1}$

i. e. , the train B appears to A to move with a speed of $40m{s}^{-1}$ from north to south.