Picture a row of people walking shoulder to shoulder down a lane. Their chests are all on the same line, and everyone faces the same way. Nobody is in front and nobody has dropped back. That single picture — level, side by side, moving together — can be taken as the thread behind every use of abreast, whether it is bikes on a road, a boat coming up beside an island, or a person keeping level with the news.
The source records abreast as Middle English abrest, formed from a- (on, at) plus breast — roughly breast-by-breast, chests in line and equally advanced. That literal shape sits right on top of the modern picture of a level row.
資料記的是：abreast 來自中古英文 abrest，由 a-（在…上）加 breast（胸口）組成，字面大致是「胸口對胸口」——一排胸口對齊、前進得一樣多。這個字面正好跟今天「齊頭並排」的畫面疊在一起。
Wiktionary (kaikki:3f6f69a190036ae4:13513247:20836), etymology of abreast (adv)
Two branches
位置上的並排與齊平
這一組直接就是核心畫面本身：東西在空間裡排在同一條線上。差別只在於，一個是「一排有幾個」，一個是「我追到跟你一樣的位置」。
r · C1
Side by side in one line, all facing the same way, as when people walk, ride or fly next to each other. A number in front of it says how many are in the line: two abreast, four abreast.
並排
消息上的跟齊
把並排的畫面從身體搬到腦袋裡：一邊是一直在變的新消息，一邊是你知道的事情。兩邊齊頭前進，你就沒有落後。這是一條教學上的連結，不是在說歷史上先有哪個用法。
r · C1
To go on learning the newest facts about a subject or a situation, so that what you know stays level with how things are right now. It is used in the fixed shape keep abreast of or stay abreast of.
跟上最新狀況