Section 261.5 - Late completion of timely investigation(a) A decision may be revised after the applicable time period in § 261.2(a) or § 261.2(b) of this part expires if the Railroad Retirement Board begins an investigation into whether to revise the decision before the applicable time period expires and the agency diligently pursues the investigation to the conclusion. The investigation may be based on a request by a claimant or on action by the Railroad Retirement Board.(b)Diligently pursued for purposes of this section means that in view of the facts and circumstances of a particular case, the necessary action was undertaken and carried out as promptly as the circumstances permitted. Diligent pursuit will be presumed to have been met if the investigation is concluded and, if necessary, the decision is revised within 6 months from the date the investigation began.(c) If the investigation is not diligently pursued to its conclusion, the decision will be revised if a revision is applicable and if it is favorable to the claimant. It will not be revised if it would be unfavorable to the claimant.