Only Moody's Investors Service, in a move on Tuesday, has reacted positively to the Burlington Northern Purchase. Moody's affirmed the Aa2 (Excellent) long-term issuer rating of Berkshire and the Aa1 insurance financial strength ratings of its flagship reinsurance companies, National Indemnity Company and Columbia Insurance Company.

Best said financial strength ratings of "A-double-plus" (superior) and issuer credit ratings of "triple-a" of National Indemnity Group and GEICO and their members were under review with negative implications.

Best also placed FSRs of "A-double- plus," and ICRs of "double-a-plus" of General Re Group and its members, under review with negative implications.

The rating firm noted that the transaction is valued at $44 billion and included the assumption of $10 billion of Burlington Northern debt.

Best said its action reflects concerns over utilization of Berkshire's "insurance and reinsurance operations as a funding source for this transaction."

Additionally, Best said it is concerned with the liquidity and concentration risk pertaining to a single large investment as a substantial portion of Berkshire's portfolio, noting the majority of Berkshire's equity investments "are held at the insurance and reinsurance entities."

The company said it will assess the under-review status over the near term as more details regarding the capitalization and liquidity of the insurance and reinsurance operations is presented by management.

S&P said the Berkshire insurance operations over the past 12 months have been buyers of the company's investments in Goldman Sachs, General Electric Co., WM. Wrigley Jr. Co., and Swiss Re.

"These large investments have attractive coupons and are boosting investment income, but have also increased the exposure of the insurance companies' statutory capital to equities and speculative-grade bonds," said S&P.

It said the investments have also reduced insurance company liquidity, as Berkshire primarily paid for them with cash and short-term investments and not from the sale of longer-term investments.

Berkshire's insurance companies already own a substantial amount of Burlington Northern stock, so any further share purchases will increase the concentration risk associated with having a substantial portion of invested assets in securities of one company, S&P found

In April, before Tuesday's action, Moody's downgraded Berkshire from its top "Aaa" rating, and Fitch reduced Berkshire to "double-A-plus" from "triple-A" in March.

According to Moody's, the railroad purchase would enhance Berkshire's earnings and cash flows, while further diversifying the portfolio of owned businesses."

Moody's did say that, near term, the transaction would reduce Berkshire's financial flexibility, given that a majority of the consideration would be in cash. But it took the view that Berkshire will maintain a large cash balance and a conservative financial profile, consistent with its long-standing practice.

In July, Berkshire Hathaway, the largest Moody's shareholder, said it had reduced its stake to 16.98 percent from 20.4 percent.