$ 解：(1)v_{ }=\frac {s_{ }}{t_{ }}=\frac {{ 900 }\ \text {km}}{{ 9 }\ \text {h}}={ 100 }\ \text {km/h}$
$ (2)s_{ }'=v_{ }'t_{ }'={ \frac {144}{3.6} }\ \text {m/s}×{ 25 }\ \text {s}={ 1000 }\ \text {m}$
$ s_车=s'-s_桥=1000\ \text {m}-400\ \text {m}=600\ \text {m}$
