A train running with a speed of $60 \text{km/hr.}$ crosses a train going with $42 \text{km/hr.}$ in the same direction. A person sitting in the faster train observes that it had taken $1.2$ minutes to completely cross the slower train. Find the length of the slower train.
- $420$ meters
- $540$ meters
- $360$ meters
- $280$ meters