To find the speed of the Acela Express in miles per minute, you can use the formula:
\[ \text{Speed} = \frac{\text{Distance}}{\text{Time}} \]
In this case, the distance is 135 miles, and the time is 60 minutes. Plugging in these values:
\[ \text{Speed} = \frac{135 \text{ miles}}{60 \text{ minutes}} = 2.25 \text{ miles per minute} \]
Therefore, the speed of the Acela Express on this portion of the route is 2.25 miles per minute.