The following message to MARC riders from Maryland Transportation Secretary Beverly Swaim-Staley was posted on the MTA's web site this morning:

Severe heat, like that forecast throughout this week, can cause problems with equipment operating on all MARC lines.  The performance of locomotives, air conditioning in passenger cars and catenary lines carrying electrical power on the Penn Line are pushed to the limit in excessive heat.  We remind customers that heat related issues have the potential to create delays throughout the system.  We want to assure you that we work with our partners (Amtrak on the Penn Line and CSX on the Brunswick and Camden lines) to aggressively address these issues when they occur and resolve them as quickly as possible.  We appreciate your understanding and your patience. 

A heat order, slowing trains because of rail conditions, has already been issued by CSX for the Brunswick Line, and the Camden Line can't be  far behind. Such orders are routine this time of year.