It can be.
A bus driver costs about $20/hour. Let's assume a train driver is paid the same for comparison purposes.
We want to move 120-150 riders every 15 minutes across a four mile distance.
We know the train will cost about $300 million right now. A train lasts roughly 3x as long as a bus if I remember right and costs $4.5 million or so. We need six trains with extras = $27 million
Each bus lasts about 12 years and costs $1 million or so. x3 for the same service So we need $54 million worth of busses to hold the same number of people as the train in the same time period. (35 or 36 years)
Say we have 112 hours of operational service to cover. So that's four drivers at once for 448 hours of coverage with the train per week
With the bus we need 3x the drivers, so that's 1344 hours of coverage with the bus per week
I gave 3% raises annually
the labor cost for the train is $1.179 billion dollars
The labor cost for the bus is $3.537 billion dollars
For a total cost over 35 years
Train at 150 people: $1,506 billion
Bus at 120 people: $3,591 billion.
I left out the cost of maintenance, benefits, electricity in the equation but maintaining more busses comes with a larger cost of course.
The problem is the up front capital costs of the train is so high but the cost of labor for the same service level is so much higher.
Now, there's larger busses that hold more people but trains scale up to much bigger numbers
For close to the same cost across 35 years we can run a train that holds 300 people.
London spent a fortune building crossrail with 1500 person trains because no matter where you count from there's always a point where rail is cheaper for the same time if the ridership is bigger than the smallest train.