Under what conditions is intermodal transportation advantageous?

In general, it's about time or money. Some modes are better for short
or long long distances. If the benefit of combining the two modes
strengths is greater than the "transfer penalty," Some examples to
ponder

1. An envelope with a contract must go from Gary, Indiana to Santa
Monica California. An express parcel service (e.g. Fed Ex) ships the
envelope by truck to Chicago, it is then flown to Los Angeles and
driven in another truck to Santa Monica. The envelope is delivered
much more quickly than if it had been driven the entire distance.

2. A case of video game consoles must be transported from a
manufacturing facility in Japan to a retailer in Cleveland Ohio.
Although it could be shipped by boat (cheap, but very slow), or flown
between the two cities (faster, but expensive), the most likely route,
balancing cost and time would be
to send a full container of 50 cases by boat from Japan to Seattle
(cheapest over water), train from Seattle to Chicago (more direct than
boat, cheaper than air, more cost effective than truck), then by truck
from Chicago to the retailer's distribution center in Columbus, Ohio
(which is not served by water, and not readily accessible by train),
where the cases are put on truck delivering cases and other items to
six stores in Cleveland.

3. An office worker travels from Oak Park, Illinois to an office in
downtown Chicago. She rides a bicycle to a rapid transit station, then
takes a train downtown. The intermodal trip is less expensive than
paying for parking a car downtown, and faster than cycling the entire
distance.

The U.S. Department of Transportation has a service that may help you.  Please see:

http://ntl.bts.gov/faq/index.html

You can search all DOT web pages using the DOT Search Engine 
http://search.google.dot.gov/DOT/DOTSearchProcess.asp

or you can submit your question by e-mailing librarian@bts.gov.

When one mode of trasportation will not suffice. The product is loaded
into an ocean container in Hong Kong and shipped to Oakland, the
container is placed on a chassis and a trucker takes it to a rail
terminal where it is loaded on a train to be taken to Salt Lake City
where it is dropped at a logistics warehouse to be distributed via LTL
truckers to various retailers.