Watching “The Shield” is a lot like sitting on a sunny hillside watching two trains rush toward each other on the same track. The fact that you know a destructive collision is inevitable doesn’t make watching for it any less exciting.

Detective Vic Mackey (Michael Chiklis) has been at the helm of one of those steam engines for five seasons. Season 6 finds him frantically trying to figure out who keeps stoking the fire while he’s working as hard as he can to slow down and avoid disaster.

This season was to have been the last for the ground-breaking show that put the FX network on the map as a bold innovator in dramatic series television. In the midst of production on Season 6 last June, the show’s creator, Shawn Ryan, agreed to extend the run for one more year. Ten episodes are slated for this season, with 13 planned for next.

Just as Mackey’s Strike Team mourns the loss of one of its own, Detective Curtis “Lemonhead” Lemansky (Kenneth Johnson) in the opening episode, the makers of “The Shield” were grieving over the death of executive producer Scott Brazil. He died at the age of 50 last April of complications from ALS and Lyme disease while the first episode of Season 6 was being shot. Brazil, who also directed 11 episodes of “The Shield,” is perhaps as tall a ghost behind the scenes as Lem is in the barn.

Besides the hand-held cameras and the slice-and-dice editing, what set the show apart from other cop series six years ago was the way it pinned viewers to their seats and made them gasp for air. Whatever the reason, the drive and energy of that first season are back in equal and welcome proportions.

Things slowed a bit in Season 2 and nearly came to a halt the next year when too much time and energy was wasted on the detectives’ personal lives and on the political ambitions of Capt. David Aceveda (Benito Martinez). The show reached new heights in Season 4 when Glenn Close guest-starred for a season as Capt. Monica Rawling.

She challenged Mackey intellectually and forced him to find new strategies for turning an enemy who happened to be female into an ally. The big-city politics that proved to be Rawling’s undoing grew more interesting and complex that year.

Last season was all about Lem as Forest Whitaker joined the cast to play an internal affairs investigator. The Academy Award winner returns for the first two episodes of the new season as Lt. Jon Kavanaugh.

The actor’s gutsy portrayal of Mackey’s obsessed moral opposite comes full circle and it’s a relief to see him go. Whitaker is a movie star in the old-fashioned sense of the word. As good as he was in the role, the TV screen is just too small to hold him.

Mackey’s nemesis this season is a former colleague – Capt. Claudette Wyms (CCH Pounder). She is his equal as a cop and can go head-to-head with him without losing her moral center. Her advantage is not that she’s smarter or more righteous than he is, it’s that she knows him so well and can predict which way he’ll zig when she or anyone else zags.

(A special 15-minute Web-only episode, “The Shield 5.5,” is available online at AOL, Heavy.com, Yahoo, MSN, TV.com, IGN, Maxim.com and Stuff.com.)

So, settle in for the rush of the unsettling ride that is Season 6 of “The Shield.” And don’t close your eyes because those engines are hurtling closer by the second and you don’t want to miss the explosive impact.