Summary

Mahlon Pitney Louisville Bridge Company v. United States…

Indeed, the proviso seems to have been so interpreted by the recipients of the grant, for, as appears from the stipulation, the original builders of the bridge did not limit themselves to giving only what they were compelled by law to give, but, at large expense to themselves, exceeded the heights and widths that the act required.
It is true that Congress must have contemplated that a large investment of private capital would be necessary, and that the bridge when once constructed could not be abandoned or materially changed without a total or partial loss of value.
Source: Wikisource

Mahlon Pitney Louisville Bridge Company v. United States…

And since our interstate and foreign commerce is a thing that grows with the growth of the people, and its instrumentalities change with the development and progress of the country, it was not natural that Congress, in enacting a regulation of such commerce, should intend to put shackles upon its own power in respect of future regulation. The act declared that the bridge, when erected, should be 'a lawful structure;' but there are no words of perpetuity, nor any express covenant against a change in the law.
Source: Wikisource

Mahlon Pitney Louisville Bridge Company v. United States…

Clearly, the acts were passed under the power of Congress to regulate commerce. That power is a very great power, and in its nature continuing, not being exhausted by any particular exercise. We need not go so far as to say that Congress could not in any case, by contract or estoppel, prevent itself from modifying or revoking a regulation once made and substituting another in its place without compensation.
Source: Wikisource

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