Same Insiders, Two Companies
Crédit Mobilier was the construction company building the railroad for Union Pacific. The problem was that many of the people making money through Crédit Mobilier were also stockholders, directors, or promoters of Union Pacific.
A normal railroad had reason to negotiate construction costs down while a contractor had reason to push them higher. Here, some of the people deciding what Union Pacific would pay also benefited when Crédit Mobilier was paid more. The conflict of interest, more than profit by itself, is what made the arrangement different from a normal business contract.
Union Pacific
Approved and paid construction contracts.
Overlapping Insiders
Could influence both sides of the arrangement.
Crédit Mobilier
Received construction payments and distributed profits.

Sources: Wilson Report, 1873, 14–18; Paul Kens, “The Crédit Mobilier Scandal and the Supreme Court.”