The distance between Mumbai and Pune is 192 km. Travelling by the Deccan Queen, it takes 48 minutes less than another train.

Let the speed of the Deccan Queen be $x \mathrm{~km} / \mathrm{hr}$.

According to the question:

Speed of another train $=(\mathrm{x}-20) \mathrm{km} / \mathrm{h}$

$\therefore \frac{192}{x-20}-\frac{192}{x}=\frac{48}{60}$

$\Rightarrow \frac{4}{x-20}-\frac{4}{x}=\frac{1}{60}$

$\Rightarrow \frac{4 x-4(x-20)}{(x-20) x}=\frac{1}{60}$

$\Rightarrow \frac{4 x-4 x+80}{x^{2}-20 x}=\frac{1}{60}$

$\Rightarrow \frac{80}{x^{2}-20 x}=\frac{1}{60}$

$\Rightarrow x^{2}-20 x=4800$

$\Rightarrow x^{2}-20 x-4800=0$

$\Rightarrow x^{2}-(80-60) x-4800=0$

$\Rightarrow x^{2}-80 x+60 x-4800=0$

$\Rightarrow x(x-80)+60(x-80)=0$

$\Rightarrow(x-80)(x+60)=0$

$\Rightarrow x=80$ or $x=-60$

The value of $x$ cannot be negative; therefore, the original speed of Deccan Queen is $80 \mathrm{~km} / \mathrm{hr}$.