5. The train is $400 \mathrm{~m}$ long, and it takes 10 minutes to pass through the tunnel (from the front of the train entering the tunnel to the rear of the train leaving the tunnel). If the speed increases by 0.1 kilometers per minute, then it will take 9 minutes, and the length of the tunnel is $\qquad$ $ـ$.
5. 8600 meters.

Let the full length be $x$ meters, and the original speed of the train be $v$ meters per minute. According to
$$
\left\{\begin{array}{l}
\frac{x+400}{v}=10, \\
\frac{x+400}{v+100}=9 .
\end{array} \text { Solving, we get } v=900\right. \text { (meters/minute.) }
$$