difference that would ordinarily impart a substantial impact to a passing rail car to correct this differential a pad 10 may be positioned beneath the concrete ties on the bridge where the pad 10 has a modulus that reduces the total track modulus of the concrete tie bridge preferably to between 5000 and 6000 likewise with a wood tie bridge and a wood tie approach a corresponding pad 10 positioned beneath the wood ties on the bridge would preferably reduce the modulus of the track from about 6800 to somewhere between 3000 and 5000 it should be noted that although the problem to be corrected typically involves a track deflection on the structure that is too high in relation to the surrounding approach care should be taken that the modified track modulus with the pad 10 is not too low as this also would create an undesirable impact or vibration the particular values shown in fig2 for the respective track modulus of the concrete and wood tie bridges and approaches are exemplary only and may vary for each particular bridge and approach depending on the construction of the bridge and the type of surrounding terrain as stated previously if the pad 10 is made of a relatively hard material e g has a durometer greater than about 65 the pad 10 will not tend to stiffen much over time as it is used however though less so than corresponding softer pads the pad 10 will likely stiffen slightly therefore it may be desirable for the pad 10 to have a modulus calculated to bring the track modulus of the bridge or other structure to about 1000 less than the.