Answer
$L = 87.2~m$
Work Step by Step
We can find $\gamma$:
$\gamma = \frac{1}{\sqrt{1-\beta^2}}$
$\gamma = \frac{1}{\sqrt{1-0.900^2}}$
$\gamma = 2.294$
We can find the length of the tunnel in the train view:
$L = \frac{L_0}{\gamma}$
$L = \frac{200~m}{2.294}$
$L = 87.2~m$
