What does this mean?

From the museum, proceed on Chestnut Avenue to Highbridge Road and cross the railroad tracks. Turn around cross the tracks and you'll see the signal on your left. (It's difficult to see when crossing the tracks the first time because the railroad curves to the left.)

The signal is Approach Restricting. The definition is Proceed prepared to stop at the next signal. Trains exceeding Medium Speed must begin reduction to Medium Speed as soon as the engine passes the Approach Restricting signal.

Restricting speed is defined as follows from Amtrak's operating manual:

80. Movement at Restricted Speed
Movements made at Restricted Speed must apply the following three requirements as the method of operation:
1. Control the movement to permit stopping within one half the range of vision short of:
a. Other trains or railroad equipment occupying or fouling
the track,
b. Obstructions,
c. Switches not properly lined for movement,
d. Derails set in the derailing position,
e. Any signal requiring a stop.

AND

2. Look out for broken rail and misaligned track.

AND

3. Do not exceed 20 MPH outside interlocking limits and 15 MPH within interlocking limits. This restriction applies to the entire movement, unless otherwise specified in the rule or instruction that requires Restricted Speed.

Pay particular attention to 1c about switches and go to the next photo.