Excessive locomotive cab noise is a real problem. Below is an article highlighting the effects of excessive cab noise.

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FRA standard  229.121 Locomotive cab noise.

After August 31, 1980, the permissible exposure to a continuous noise in a locomotive cab shall not exceed an eight-hour time-weighted average of 90dB(A), with a doubling rate of 5dB(A) as indicated in the table. Continuous noise is any sound with a rise time of more than 35 milliseconds to peak intensity and a duration of more than 500 milliseconds to the time when the level is 20dB below the peak.

 
------------------------------------------------------------------------
                                                                 Sound
                 Duration permitted (hours)                      level
                                                                (dB(A))
------------------------------------------------------------------------
12..........................................................          87
8...........................................................          90
6...........................................................          92
4...........................................................          95
2...........................................................         100
1\1/2\......................................................         102
1...........................................................         105
\1/2\.......................................................         110
\1/4\ or less...............................................         115
------------------------------------------------------------------------

When the continuous noise exposure is composed of two or more periods of noise exposure of different levels, their combined effect shall be considered. Exposure to different levels for various periods of time shall be computed according to the following formula:

D=T1/L1+ T2/L2+. . . . TN/LN

where:

D = noise dose.

T = the duration of exposure (in hours) at a given continuous noise level.

L = the limit (in hours) for the level present during the time T (from the table).

If the value of D exceeds 1, the exposure exceeds permissible levels.

Exposure to continuous noise shall not exceed 115dB(A).

Noise measurements shall be made under typical operating conditions using a sound level meter conforming, at a minimum, to the requirements of ANSI S1.4–1971, Type 2, and set to an A-weighted slow response or with an audiodosimeter of equivalent accuracy and precision.

In conducting sound level measurements with a sound level meter, the microphone shall be oriented vertically and positioned approximately 15 centimeters from and on axis with the crew member's ear. Measurements with an audiodosimeter shall be conducted in accordance with manufacturer's procedures as to microphone placement and orientation.

Additional links and resources:

Locomotive Safety Standards

http://www.volpe.dot.gov/opsad/lococab/lococh3.html#top

http://www.asha.org/public/hearing/disorders/noise.htm

http://www.nohsc.gov.au/OHSInformation/NOHSCPublications/factsheets/nois21.htm